Sunday Oct 11, 2009 at 22:12

TPE 1.0 Released

Brief update to say that The Photographer’s Ephemeris is now officially out of Beta!

The new version is available for download now and current users will be prompted to auto-update.

The desktop version of TPE remains free of charge and the program will not expire.

Posted in

Stephen · Sunday, October 11, 2009, 22:12 · Permalink

Sunday Sep 20, 2009 at 13:57

TPE updated to 1.0 RC1

I’ve updated TPE today to version 1.0 RC1 – it’s close to done (for version 1 at least).

A couple of features of note:

Elevation

I’ve been using the SRTM (Shuttle Radar Topography Mission) data set via the Geonames web service. This is a fairly mature data model and the quality is high. However, it has a couple of limitations for our purposes.

Firstly, the latitudes of the model extend only to 60° north and 56° south – not far enough for some of the locations that photographers get to nowadays.

Secondly, many mountainous peaks and slopes would return “No Data” due to measurement difficulties when the angle of incidence of the radar was too high. Again, many landscape photographers are interested in precisely these locations.

In this version we’re adding two other elevation data models to the mix: ASTER GDEM and GTOPO30.

ASTER GDEM was released only earlier this summer, but Geonames already has a web service supporting it. The coverage is much wider (83° north and 60° south) and mountainous areas are better covered.

However, there are apparently still some holes and data anomolies in the model (see the reference in the Wikipedia article linked above).

GTOPO30 is another model with even wider coverage but with lower resolution and the data is older than the other two sources.

So, in the new version of TPE, I query SRTM first and then ASTER and finally GTOPO30 if either preceding service returns “no data” (or the location is out of range).

Twilight azimuths

I had a request from Tim Parkin to display the azimuth lines for the sun both before sunrise and after sunset. This is useful if you’re looking to track the twilight glow on the horizon.

So, the program now shows this (in the Details View) up until the end (or from the start) of Astronomical Twilight.

Posted in

Stephen · Sunday, September 20, 2009, 13:57 · Permalink

Thursday Aug 20, 2009 at 21:47

Using TPE, Part 4: the Horizon

Here’s the fourth in a series of tutorials on The Photographer’s Ephemeris.

We covered the basics of using the program in Part 1. In Part 2, we covered the Twilight information and the Details View (most of it at least). In Part 3 we covered the use of the secondary map marker. You’ll need to have understood the material in those tutorials before tackling this one.

This tutorial is based on Beta 0.9.6. Click on any screenshot for a full-size expanded view.

The Horizon

Why should the photographer care about the horizon? Simply put, it’s the visible boundary above which the sun or moon rises and below which they set. Knowing where that boundary lies can be important for setting up your shots.

It’s common experience that you can see farther when stood atop a mountain, a tall building or when flying in an aircraft. The distance to the visible horizon increases in proportion to your height above the ground. If you can see farther, then you’ll see the rising sun sooner than if you were back on the ground, or the setting sun later.

So, in short, height above the horizon changes the precise times of sun/moon rise/set.

TPE can adjust for height above the horizon. In this tutorial we’ll walk through the steps to accomplish that.

It’s optional

One important note: this is all optional. You don’t have to worry about it. By default, the times for rise/set that TPE gives match those of the vast majority of other online sources. Very few of these correct for height above the horizon, and you likely won’t run into many problems as a result.

The usual advice applies: arrive at your location early and be prepared to stay late. Do that, and the differences in rise/set times due to height above the horizon most likely won’t worry you.

So when does this matter?

So when might you want to worry about it? Here are some example situations:

  • Shooting sunrise from a mountain peak (e.g. looking east across the San Juans from the summit of Mt. Sneffels)
  • Shooting last light striking a mountain peak (take your pick of summits)
  • Shooting a seascape from a high sea cliff at sunrise/sunset (when and where will the sun set as seen from the 601m high Slieve League in Donegal, Ireland?)
  • You need to know how far you might be able to see from a high point on the landscape (e.g. can I see Shiprock, New Mexico from Mesa Verde, Colorado?)

Let’s discover how.

Back to the Rockies

We’re going to return to the Dream Lake location we used in Tutorial 1.

TPE Lesson 4 Screenshot 1

  1. The quickest way to get back there (assuming you didn’t already save it as a Location) is to search for “Tyndall Gorge, Colorado”
  2. This should drop you just a little east of Dream Lake
  3. Set the date to September 12 2009

Next, let’s position ourselves a bit better for where we’re planning to shoot from:

TPE Lesson 4 Screenshot 2

  1. Drag and drop the primary map marker (red) to a point at the east end of Dream Lake, then pan and zoom to show the top of Hallett Peak and Flattop Mountain to the west

It’s going to be a sunrise shot, so let’s get our time set correctly:

TPE Lesson 4 Screenshot 3

  1. Click Details to switch display modes
  2. Click Next event to move to sunrise and then drag the slider a couple of minutes later to 06:46 hrs
  3. Drag and drop the secondary marker (grey) to a point on the eastern flank of Hallett Peak, aiming for the most tightly packed contour lines
  4. Note that the apparent altitude from lakeshore to mountain flank is 20.4°

So far so good.

But what we are really shooting?

But let’s think about it. What are we shooting here? Where will the rising sun fall?


Hallett Peak from Dream Lake

Hallett Peak from Dream Lake (this was taken in March rather than September, but the rising sun is at a similar azimuth)

Really, we should have our primary marker on the mountain sides: that’s where first light will strike – not the ground underneath our tripod. We need to reverse the marker positions. Fortunately, there’s an easy way to do that:

TPE Lesson 4 Screenshot 4

  1. Click the Swap button next to the Geodetics label (or press the S key)
  2. Note that the two markers swap positions

The point we’re photographing is significantly higher than the lake – some 2,110 feet higher. If you’ve ever been to Rocky Mountain National Park, you’d likely have noticed that you can see clear to the east for a rather long way. That’s because the plains lie several thousand feet lower in elevation. Let’s find out exactly how much lower:

TPE Lesson 4 Screenshot 5

  1. Zoom out so you can see the plains of eastern Colorado as shown
  2. Reposition the secondary marker along the sunrise azimuth line, dropping it somewhere beyond Interstate 25
  3. The Geodetics panel tells us that the change is elevation is some 7,000 feet and the distance over 40 miles

Setting the Elevation at the Horizon

This is the critical step.

Knowing that the plains are just that – plains, and therefore flat – we can use our roughly positioned secondary marker to set the elevation at the horizon. TPE, knowing the elevation at both primary and secondary marker positions, can then take the difference to calculate the elevation above the horizon, which is the number we need to adjust the rise and set times:

TPE Lesson 4 Screenshot 6

  1. Click Lock in the Elevation at the Horizon panel: this locks the elevation at the horizon to the secondary marker position (you can also manually type a value into the text box if you prefer – press Enter when you’re done). Note that the elevation above sea level at the secondary marker position is displayed in the text box.
  2. Note that the time of sunrise has changed: it’s now 06:33 rather than 06:42, and the azimuth has changed also. That nine minute difference is the effect of the elevation above the horizon when up on the flank of Hallett Peak
  3. Finally, note that the azimuth lines are now split either side of the secondary marker – although our time of day setting hasn’t changed, now that we’re accounting for height above the horizon, sunrise is sooner and occurs farther north

You may be wondering, just because we happened to drop the secondary marker just east of I25, that doesn’t mean that’s where the visible horizon is, right? Right. It’s an estimate. Remember the trial-end-error elements of Tutorial 3? This is another one of those. However, the program gives us a clue as to how close we might be:

TPE Lesson 4 Screenshot 7

  1. Zoom out a little (click on the zoom control, press Ctrl-minus, use the mouse scroll wheel) to reveal…
  2. A visual indication of the implied distance to the horizon
  3. If you hover the mouse over the horizon indicator icon, the tooltip will indicate the calculated distance to the horizon

So, if the elevation above the horizon is the 7,000 ft implied by our marker locations, we should be able to see around 111 miles to the east (from the flank of Hallett Peak). Note that the distance is an estimate, based on theoretical (but reasonable) calculations and assumes a ‘standard’ set of atmospheric conditions. See Andrew Young’s Distance to the Horizon page for more details – there’s some interesting background material here too.

Having zoomed out and seen that the implied distance to the horizon is much farther than the location we selected to determine Elevation at the horizon, it makes sense to adjust and double check:

TPE Lesson 4 Screenshot 8

  1. Drag the secondary marker to the limit of the visible horizon and release
  2. You’ll see that elevation above sea level is slightly lower again at this location, and the distance to the visible horizon increases slightly, but not significantly

Finally, let’s test a little further away and see if we can tease the horizon even further out:

TPE Lesson 4 Screenshot 9

  1. Drag the secondary marker a little further along the sunrise azimuth line
  2. The change in elevation is minor (the plains are getting pretty flat our here) and the horizon limit doesn’t follow us. We’re beyond the visible horizon and a warning icon is displayed next to the Distance and bearing label in the Geodetics panel

This time, we’ve gone too far. The secondary marker is beyond the calculated distance to the horizon. We can move it back in and call it done.

What have we achieved with all this?

Let’s review what we’ve accomplished:

  • We’re shooting sunrise on some mountain peaks that lie to the west of an extensive plain at lower elevation
  • We know that the sun will be seen from the mountain peaks earlier than it would at a lower elevation because the distance to the horizon is greater
  • If we want to correct the rise and set times for this “dip of the horizon”, we need to tell TPE what the elevation above sea level is at the horizon
  • Adopting a simple trial and error approach, we can drop the secondary marker in a likely looking location, Lock the elevation at the horizon to the secondary marker position and let the program recalculate
  • By zooming out we can see the implied distance to the horizon and use that as a hint of where to try the secondary marker next
  • With a little trial and error, we can get a decent estimate of where the visible horizon will lie
  • If you were shooting from the mountain peak (as opposed to shooting the peak itself) the distance to the horizon will show you what landscape features you might see in your shot

Gotchas

The same gotchas apply as from Tutorial 3 – you need elevation above sea level for both marker positions. However, in addition:

  • The distance to the horizon will vary depending on which direction you look in. Therefore, it’s important to establish the horizon in the direction from which the light is coming or in which you plan to shoot. (For example, above, the distance to the horizon in the east is very different to the distance to the horizon to the west.)
  • You need to pay attention to the contour information contained in the topographic map in order to make educated trial and error attempts
  • In varied terrain, you may need to test more locations than you might in flatter terrain as used in this example
  • If you need to establish the elevation at the horizon, but still wish to use the secondary marker for other purposes (e.g. as per Tutorial 3), then do the following: (i) establish the elevation at the horizon first, using the Lock function; (ii) once set, Unlock from the secondary marker – this leaves the elevation at the horizon set, but you can now move the secondary marker freely without changing it
  • If you need to the clear the elevation at the horizon, click the X button to the right of the Elevation at the horizon text box

If you’ve enjoyed this tutorial, you might also enjoy “Understanding Light with The Photographer’s Ephemeris” co-authored with renowned landscape photographer Bruce Percy. It’s available through Bruce’s web-site

Posted in

Stephen · Thursday, August 20, 2009, 21:47 · Permalink

Tuesday Aug 11, 2009 at 23:23

Using TPE, Part 3: Geodesy

Here’s the third in a series of tutorials on The Photographer’s Ephemeris.

We covered the basics of using the program in Part 1. In Part 2, we covered the Twilight information and the Details View (most of it at least). You’ll need to have understood the material in those tutorials before tackling this one.

This tutorial is based on Beta 0.9.6. Click on any screenshot for a full-size expanded view.

Geodesy?

Geodesy? Geodetics? What’s that all about? I’ll admit that until I started really getting into writing TPE, I didn’t have a clue. However, it turns out that there are whole class of questions a landscape photographer might legitimately ask that can only be answered accurately by use of the science of geodesy.

I’ll leave it to Wikipedia to explain the details, but in essence, geodesy deals with the measurement and mathematical representation of the earth.

The earth is round. Sort of. In fact, it’s sufficiently not round that measuring point-to-point distances on the surface of the earth is only poorly approximated by assuming a sphere. You wouldn’t want your airline pilot navigating this way.

An ellipsoid is a much better assumption to make, but the maths gets hard. So hard, in fact, that a decent solution for calculating point-to-point distances between points on the surface of an ellipsoid was only devised in 1975 by Thaddeus Vincenty.

The Geodetics panel and the accompanying Secondary Map Marker included in v0.9.5 of TPE and later use Vincenty’s algorithms to enable some new functionality that will help you plan shoots in greater detail.

Our destination for this tutorial: the Macey Lakes

Colorado’s Sangre de Cristo Wilderness contains some of the most spectacular peaks in the whole of the Rockies. There are around 18 drainages within the wilderness boundaries, many with stunning alpine lakes surrounded by jagged mountainous cirques.

TPE Lesson 3 Screenshot 1

  1. Type ‘Macey Lakes’ into the search box, and press Enter
  2. The primary map marker in red will (should) be positioned over the Macey Lakes in Colorado, USA
  3. For the purposes of this tutorial, set your date to July 5th 2009

We’re zoomed out a bit too far as is, so let’s fix that:

TPE Lesson 3 Screenshot 2

  1. Drag and drop the primary marker to the north east of the lower lake, as shown
  2. Zoom in around 3 clicks or so (you may prefer to zoom in before dragging the marker – do whatever works for you)
  3. Click the Details button to show the details view for July 5th
  4. Notice that we now have a Secondary Map Marker in light grey

Using that secondary marker is what this tutorial is all about.

A few things about it:

  • It’s optional – you don’t have to use at all if you don’t want to
  • By default, it will always appear to eastern side of the map
  • If you don’t drag and drop it, it stays light grey
  • Moving it won’t (by default – check back for the next tutorial) change your sun/moon rise/set/phase or twilight times

However, we won’t learn much by leaving it alone, so let’s see what useful information this could provide us.

When will I lose direct sunlight on Lower Macey Lake?

Looking at the map, you can see that the sun will set to the north west at this time of year. It’s also easy to make out the high ridge-line in the same direction, with the summit of Little Baldy Mountain clearly marked. Just eyeballing the contour lines, it seems likely that the sun will disappear behind the ridge well before it actually sets below the true horizon. But when?

We can use the secondary marker to find out.

TPE Lesson 3 Screenshot 3

  1. Start by dragging and dropping the secondary marker on the summit of Little Baldy. You’ll notice that when you do, the colour changes to a darker grey, indicating that you have activated the geodetics information
  2. In the Geodetics panel, you’ll now see three numbers displayed. The most significant for our purposes is the apparent altitude of 18.5°

What do they tell us? Firstly, notice the icon to the left. It shows an arrow from primary to secondary marker. This indicates that all the data displayed in the panel is referenced in terms of travel from the primary location to the secondary. Let’s look at the three data items in reverse order from the bottom up:

  • Distance and bearing: distance is the point-to-point as-the-crow-flies distance from primary to secondary marker; bearing is the map bearing from primary to secondary in degrees (note: this is map bearing, not compass bearing – the same comment applies to all azimuths and bearings in v0.9.6, although I will likely add a compass bearing option in a later version)
  • Change in elevation: elevation refers to height above mean sea level. The change in elevation is measured from primary to secondary. In this case, it’s 1,391ft from the lower lake to the summit of Little Baldy
  • Apparent altitude: the units of degrees give away that this is altitude in the astronomical sense. If you had a sextant and took a sighting to the peak, this is angle you would measure. ‘Apparent’ means that this measurement is adjusted for refraction, the bending of light caused by passage through the atmosphere.

(Note that the apparent altitude is not exactly what you’d get by dividing the elevation change by the distance and calculating the inverse tangent: the calculation accounts for the curvature of the earth’s surface and adjusts the result for refraction.)

OK, now we know what we’re looking at, let’s find out the altitude of the sun when it passes through the same bearing:

TPE Lesson 3 Screenshot 4

  1. Use the time of day slider to drag forward to around 18:15.
  2. You’ll see the azimuth line for the sun move around during the course of the day and line up with the grey line to the secondary marker. We already have learned something: the sun will pass through the line of the peak of Little Baldy at 18:16 on July 5th 2009. But will it be visible?
  3. Looking at the sun’s altitude in the Details panel, you can see that it lies at 23.4° some 5° above the peak of Little Baldy

So, the sun should still be visible at 18:15. We need to look a little further:

TPE Lesson 3 Screenshot 5

  1. Drag the time of day slider a little later in the day until the time is around 18:40. The sun is setting in the sky, so the altitude decreases to 18.8°
  2. Drag the secondary marker a little farther to the north-east along the ridge line to match the azimuth line to the setting sun.
  3. Note that the apparent altitude to the ridge line is now 18.8°

With a little trial-and-error, you can establish that the sun is likely to drop out of sight around 18:40, some time before actual sunset. You’ll need to apply some judgement here and look at the contours of the topological map (don’t try this in other map views) and see where the sensible test points should be. We’ll look at this in more detail below.

As an exercise, you may wish to try to determine how high on the north west flank of Colony Baldy will you observe direct light in the moments before sunset. Hint 1: you’ll need to relocate both markers. Hint 2: you may need to move the secondary marker farther than you think. Answers at the end.

Will the rising sun strike Point 13,200’?

Let’s look at a different question. You want to make a sunrise image of Upper Macey Lake, and you’d like to take in the cirque to the south of the lake. However, the image will likely only work if the tops of the cirque catch the rising sun. You can use TPE to determine if the rising sun will be obstructed or not:

TPE Lesson 3 Screenshot 6

  1. Use the skip backward button to move the timeline back to the moment of sunrise, 05:47
  2. Move the primary marker to the top of the peak near the contour label 13,200’ on the map. Next, move the secondary marker to the first ridge line to east north east as shown
  3. Notice the apparent altitude and change of elevation figures

So far, so good: the first ridge line lies below our peak by some margin, so we should get some direct light. However, to be sure, let’s check to see if Colony Baldy, farther to the east will cause us any problems:

TPE Lesson 3 Screenshot 7

  1. Move the secondary marker out towards the direction of sunrise and drop it on the high point of the flank of Colony Baldy
  2. Note the apparent altitude: it is still negative, indicating that the sun will clear Colony Baldy and strike our peaks

Good news. We should be able to make the shot. We can already see from the basic sun rise line that we should get good light over the lake itself at the moment of sunrise. Now that we know our rugged mountain ridge will also receive some direct light, we can hope for a good shot:


Upper Macey Lake

Upper Macey Lake

Can we really see the ridge line?

Let’s look once more at the ridge line visibility question. This time, let’s say we want to determine the angle of view to the ridge line to the west of the upper lake:

TPE Lesson 3 Screenshot 8

  1. Move the secondary marker to the ridge line west south west of Upper Macey Lake, opposite where the sun will rise (you’ll need to reposition the primary marker too)
  2. The apparent altitude is 23.5°

OK. But this is where some trial and error and map reading skills come in.

TPE Lesson 3 Screenshot 9

  1. Move the secondary marker down the slope a little to where the contours appear a little steeper
  2. Note the increased apparent altitude – it’s now some 3 degrees greater

It might be that we’ll be looking at a false summit from our position on the lake shore. Probably won’t impact our images significantly in this case, but it’s important to be on the look out for these details in some situations.

Why can’t the computer just work all this out for me?

Reasonable question. The main reason is that the computer would have to check out elevation data at every point along the path to the sun or moon and infer what was significant for your image and what was not. There are certainly some possibilities for taking this approach, but for now, I’ve opted for the simpler manually placed marker. That approach avoids too much second guessing by the program, avoids me having to pull too much elevation data and covers all scenarios, albeit potentially with more trial and error on occasions.

Gotchas

The Geodetics calculation can determine distance and bearing quite happily just from the map marker positions (which we always know by definition – you placed the markers). However, to do anything more, we need to know the elevation above sea level for both marker positions. Some potential gotchas:

  • The program may be unable to obtain an elevation for extreme latitudes (the Shuttle Radar Topography Mission only covered latitudes from around 60°N to 56°S)
  • Sometimes, there is no data available for very steep terrain, as you might find in mountainous areas (for example, try the summit of Longs Peak, Colorado – no data, even though it’s a famous local fourteener)
  • The elevation data points are spaced every 90 metres (3 arc-seconds), so relying on this for high precision, short distance work is not recommended

That said, for most landscape uses, this will work well. However, if you have a shot that requires critical planning, I highly recommend that you

  • Consult multiple reliable sources for sun/moon information (I highly recommend Jeff Conrad’s Sun/Moon Calculator)
  • Obtain a large-scale topographical map of the area from a reputable publisher of your shoot and take careful measurements of distance and elevation
  • Consult the online tools from the National Geodetic Survey and perform your own geodetic calculations
  • Maintain your sanguine disposition when, even though the clouds cooperated, the sun or moon did not appear quite where or when your expected: even if all your preparation and calculation was perfect, the vagaries of atmospheric refraction may result in an unexpected outcome

Answer to the exercise

I make it 13,350ft. The ridge line of Little Baldy isn’t the limiting factor – you need to look at the next ridge line farther north west which lies higher. Place the secondary marker there, and then adjust the primary marker up and down the north west flank of Colony Baldy until you obtain an apparent altitude of around zero. From that point upwards, you should see direct light from the setting sun. More or less.

The next tutorial will cover Elevation at the horizon. If you’re shooting in high places, this could be significant…

You might also enjoy “Understanding Light with The Photographer’s Ephemeris” co-authored with renowned landscape photographer Bruce Percy. It’s available through Bruce’s web-site

Posted in

Stephen · Tuesday, August 11, 2009, 23:23 · Permalink

Monday Aug 10, 2009 at 23:07

Using TPE, Part 2: Twilight and Details View

Here’s the second in a series of tutorials on The Photographer’s Ephemeris.

We covered the basics of using the program in Part 1. In Part 2, we’ll cover the Twilight information and the Details View (most of it at least).

This tutorial is based on Beta 0.9.6. Click on any screenshot for a full-size expanded view.

A visit to the Blue Lakes

First things first: we need to choose a location for the tutorial that lets us illustrate the relevant features. To get started, let’s find our location:

  1. Click into the Search text box (below the map), type “Sneffels”, and press Enter to perform the search.
  2. You should see the primary map marker (the red one) at the summit of Mount Sneffels, one of Colorado finest fourteeners (summits over 14,000ft)
  3. If for some reason, you don’t end up there, try searching for “Mount Sneffels, Colorado, USA
  4. My date is set to August 3rd 2009 – that is significant for some of the illustrative use cases discussed below. You can set your date the same if you’d like using the date selection control.

If you’re following along, your screen should appear as follows:

TPE Lesson 2 Screenshot 1

  1. The search box shows the search term you entered
  2. The primary map marker is positioned over the closest matching location (the summit of Mount Sneffels in this case)

Let’s pan the map a little to the south west, towards the highest of the three Blue Lakes. (Why there? Why then? I was there – you can see some of the images here)

Next, click on the Twilight button towards the lower right of the screen:

TPE Lesson 2 Screenshot 2

Viewing twilight information

TPE Lesson 2 Screenshot 3

When you click Twilight, the normal sun/moon rise/set information is replaced with Twilight information. The three standard twilights are shown: astronomical, nautical and civil, as well as sunrise and sunset information.

You can click on any of the labels (e.g. “Nautical”) to display a Glossary entry for the term. In brief summary, however, astronomical twilight occurs when the sun lies between 12° and 18° below the horizon; nautical when the sun lies between 12° and 6°; and civil when the sun lies between 6° and 0°.

You can toggle the display back to the normal sun/moon display by clicking the button again. Alternatively, the display can be toggled using the ‘T’ key on the keyboard.

Getting down to the details

Next, click the Details button (or hit the ‘D’ key on the keyboard):

TPE Lesson 2 Screenshot 4

The details view is displayed. This includes a number of information panels.

  1. Click the Multi-day button (or press ‘D’ again) to revert to the normal view
  2. In Details view, the top two panels display both sun/moon rise/set/phase information, plus the twilight information discussed above, for the selected date
  3. The central panel shows graphs of the altitude of the sun and moon over the course of the selected day, plus some time-specific data and other controls (discussed below)
  4. Finally, there’s now an additional marker shown on the Map. This relates to the Geodetics panel, but we’ll discuss this in a later tutorial

(Note that the term ‘altitude’ is used in the astronomical sense of angle above the horizon and is displayed in degrees. Elevation is used to refer to height above mean sea level.)

Why would you want to know about Twilight?

Any number of reasons: many photographs, such as landscapes including mountain alpenglow are photographed during times of twilight.

Let’s take a very practical example from our Blue Lakes location on Aug 3 2009. Imagine you wanted to do some night photography of the Blue Lakes and surrounding mountains with a clear, starry sky overhead. When would be a suitable time to shoot that, during the night of August 3rd?

We can use the Details view and Twilight information to find out.

TPE Lesson 2 Screenshot 5

  1. The skip forward and backward buttons in the central panel allow you to jump through the timeline of the selected 24 hour period from ‘celestial event’ to ‘celestial event’, e.g. from Moonset (the default starting time on this particular day in this particular location) to the start of Astronomical twilight.
  2. If you click Skip forward (the button on the right), the label “Astronomical twilight begins” appears
  3. You’ll see that at this point in time, the altitude of the sun is -18° (by definition)
  4. Finally, note that the timeline indicator (and the manual slider control) have moved forward to show the altitude of the sun and moon at the corresponding time

If you’re looking to shoot a clear, starry sky, you probably want it to be truly dark. Using the information in the details panel, you can see that on this particular night, there’s only a small window of opportunity: the moon sets at 03:53 am, but astronomical twilight begins at 04:33 am. It’s likely that the best time is somewhere between 04:15 and 04:30am. (Once astronomical twilight begins, objects such as the Milky Way will become invisible in the sky.)

The path of the sun

Let’s imagine, being a glutton for punishment as most landscape photographers are, that you plan to return to the upper Blue Lake for a sunset shoot that evening. You know that you need to be there a bit before actual sunset in order to catch the sun before it drops behind the mountains to the west (there’s a good way to find out when this will be – check back for the next tutorial). But exactly what angle will the sun be at, let’s say, 45 minutes before sunset?

TPE Lesson 2 Screenshot 6

  1. You can use the slider control to set your time of day manually – here I’ve dragged the control to 7:31 pm
  2. As you move the slider, if the sun or moon lies above the horizon, an azimuth line is drawn on the map to show the bearing. (The length of the line is proportional to the altitude also – if the sun is high in the sky, the line will be short. This indication is not to any fixed scale, and is indicative only.)
  3. If you hold down the Shift key, in Details view, it is the individual azimuth lines that are extended as opposed to the rise/set lines. This lets you gauge where the light will fall relative to the primary map marker location

Other practical uses

So, we’ve covered Twilight times and most of the Details view. Twilight information is useful for many purposes. Remember the actual length of twilight varies significantly by season and by latitude (short in the tropics, long in the polar summer).

The effects of twilight on photography are important for many landscape compositions. At temperate latitudes, such as here in Colorado (40°N) late Nautical and early Civil twilight often offer more intense sky colours than late Civil twilight. Alpenglow will typically last until 10-15 minutes before sunrise – roughly mid-way through the typical Civil twilight period.

Often, you might wish to find the time of alignment of the sun or moon with a particular object or landscape feature: the manual slider control and azimuth lines allow you determine alignment visually, for example you can experience Manhattanhenge in Toronto on October 25th 2009 at 4:18pm.

Next time we’ll explore the remaining panel in Details View, the Geodetics panel.

You might also enjoy “Understanding Light with The Photographer’s Ephemeris” co-authored with renowned landscape photographer Bruce Percy. It’s available through Bruce’s web-site

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Stephen · Monday, August 10, 2009, 23:07 · Permalink

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An Englishman living in Colorado since 2007, photography has proven the perfect way to start exploring the American Southwest.

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