Using TPE Desktop Web App, 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 went beyond the basics. In Part 3 we covered the use of the secondary map marker (the grey pin) and looked at geodetics. You’ll need to have understood the material in those tutorials before tackling this one.

This tutorial is based on Beta v0.9.10. 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 or the setting sun later than if you were back on the ground.

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 quite advanced, meant for specific terrain and entirely optional. By default, TPE’s times for rise/set match those of the vast majority of other online sources, where rise/set is stated from ground level to the ideal horizon. Very few of these correct for height above the horizon.

You likely won’t run into many problems as a result of not using this feature. In fact, the usual landscape photographer’s sound 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 won’t concern you.

So when does this matter?

Here are some example situations when knowing about the horizon will help your photography planning:

  • 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?)
  • You are planning a shot that requires precise alignment between sun/moon and an object in the landscape, and you’re shooting from a point high above the surrounding terrain

Let’s discover how this works in TPE.

Back to the Rockies

Search for “Tyndall Gorge, Colorado” (yes, we’re back in Rocky Mountain National Park). You should see Dream Lake just slightly to the west.

When setting up a pin position it is sometimes a good idea to see a bit more map. You can do this in TPE by using the blue buttons above the map to the right. The first toggles the timeline on and off and the second toggles full screen on and off. Full screen mode hides or reveals both the timeline and chart, and uses a widescreen layout, but the time slider and legend remain visible. It is simpler still to use the keyboard shortcuts: T to toggle the timeline or F to toggle full screen mode.

Next, let’s position ourselves for the shot: drag and drop the primary map marker (the red pin) to a point at the east end of Dream Lake.

Set the date to September 12 2009. Click the calendar icon, click the month name at the top, click the year number at the top, click 2009, September, 12.

Note: no, your eyes are not deceiving you, (and no, TPE is not broken) there is no moonrise for this date, so no moonrise azimuth is shown.


  1. The red pin is positioned at the east end of Dream Lake
  2. The date is set to 12 September 2009
  3. Timeline toggle button. Or use the keyboard shortcut: T
  4. In this view full screen mode is toggled on. Use the full screen toggle button or the keyboard shortcut: F

If you have been in full screen mode or have the timeline hidden, toggle both so that the timeline and chart are visible.

We now need to see the top of Hallett Peak (to the west of Dream Lake) as well as the red pin. You may need to zoom out and move the map to get both points visible on your screen.

It’s going to be a sunrise shot, so let’s set our time correctly. Click the sunrise event in the timeline. The time slider and legend will jump to the moment of sunrise for the position of the red pin and the chosen date: 06:41

Let’s advance the time by 5 minutes. You can do this by scrubbing the time slider, but a more accurate way is to advance minute by minute. Click on the slider (it highlights blue) then use your keyboard cursor keys to advance the time to 06:46.

Click the grey pin button to engage the geodetics function or use the keyboard shortcut: G. Drag and drop the secondary marker (the grey pin) to a point on the eastern flank of Hallett Peak, aiming for the most tightly packed contour lines

Note that the apparent altitude from lakeshore to mountain flank (from my red pin to my grey pin) is +20.30°


  1. The time is set to sunrise by clicking the sunrise event in the timeline
  2. The time was advanced to 06:46 by clicking on the time slider then using the keyboard cursor keys
  3. The geodetics function button. Or toggle it on and off using the keyboard shortcut: G
  4. The apparent altitude from the red pin to the grey is +20.30°

But what we are really shooting?

Perhaps it’s time to think about what we are planning to shoot here. Where will the rising sun fall?

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 red pin on the mountainside: that’s where first light will strike – not the ground under our current tripod position. We need to reverse the pin positions. Fortunately, there’s an easy way to do that:

Click the Swap button on the map or use the keyboard shortcut: S – the red and the grey pins swap positions.

Remember, information in TPE is always expressed for the position of the red pin, and the geodetics information always shows travel from the red pin to the grey pin. Note how the information has changed in the geodetics panel: the change in elevation and altitude numbers are now negative and the sun and moon apparent altitudes have changed slightly to match the new red pin position for the time slider’s selected time.


  1. Use the swap button to swap the postions of the red and grey pins. Or use the keyboard shortcut: S

The point we’re photographing is significantly higher than the lake – over 2000 feet. From Rocky Mountain National Park you can see clear to the east for a rather long way because the plains lie several thousand feet lower in elevation. Let’s find out exactly how much lower.

Zoom out so you can see the plains of eastern Colorado as shown. Move the grey pin out along the sunrise azimuth line, dropping it somewhere beyond Interstate 25/highway 87.

The Geodetics panel tells us that the distance between the pins is over 40 miles and that the change in elevation is more than 7,000 feet. The azimuth from the red to the grey pin is 84.20° the same as the sunrise azimuth.


  1. The grey pin is moved out onto the eastern plain along the sunrise azimuth
  2. The darker sun azimuth, just south of the sunrise azimuth, shows our time slider set time of 06:46
  3. The distance between my red pin and grey pin is 40.010 miles
  4. The change in elevation is 7,175 feet
  5. The azimuth from the red pin to the grey is 84.20°
  6. Sunrise is at 06:41 at an azimuth of 84.2°

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 grey pin to set the elevation at the horizon. TPE knows the elevation at both red and grey pin positions, so can take the difference to calculate the elevation above the horizon, which is the number we need to adjust the rise and set times.

Look at the top of the map in the centre. You will see the words “Not set” and next to that the horizon lock.

Click the horizon lock: this locks the elevation at the horizon to the grey pin position. Note that once the horizon lock is engaged “Not set” has changed and now shows the elevation above sea level at the grey pin position.

You can also manually type a value by clicking on “Not set”, typing in your chosen value and then hitting return.

To delete a value from the horizon text box: click on the value, delete the value and then hit return.

Now look at the time of sunrise, it has changed to 06:33 rather than 06:41. The sunrise azimuth has changed also, it is now further north of our grey pin – and, 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 to the north.

That eight minute difference is the effect of the elevation above the horizon when up on the flank of Hallett Peak


  1. Engaging the horizon lock sets the elevation at the horizon to the grey pin position
  2. While the grey pin azimuth remains the same, the sunrise azimuth is now farther north
  3. The sunrise time is now eight minutes earlier at 06:33 and the azimuth is now 82.8°

You may be wondering, just because we dropped the grey pin somewhere east of I25, is that where the visible horizon actually is? Well no: it’s an estimate. Remember the trial-end-error elements of Tutorial 3? This is another case where trial and error produces an acceptable result. However, the program gives us a clue as to how close we might be.

Zoom out a little on the map (you may need to zoom out 2 or 3 times).

You will see a grey circle centred on the red pin. This is the horizon overlay and is a visual indication of the implied distance to the horizon. The calculated distance to the horizon from the red pin’s position is indicated next to the horizon lock, above the map. For my red pin position it stands at ~112 miles.

The rise and set azimuths look quite stunted from this far out. TPE limits all azimuths to 200 miles, except for the pin azimuth. Note: as stated before, there is no moonrise on this date, so no moonrise azimuth is shown.


  1. The calculated distance to the horizon for my red pin position is ~112 miles
  2. The horizon overlay is a visual indication of the implied distance to the horizon
  3. You can see the ends of the azimuth lines when zoomed out this far

So, from the flank of Hallett Peak, if the elevation above the horizon is over 7,000 ft (implied by our pin locations), we should be able to see ~112 miles to the east. It’s important to understand 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 out along the plain than our first trial and error placement of the grey pin, it makes sense to adjust and double check.

Place the grey pin out along the sunrise azimuth, where it intersects with the visible horizon overlay circle.

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


  1. The grey pin has moved out along the sunrise azimuth to where it intersects with the horizon overlay
  2. The distance to the visible horizon number has changed slightly
  3. The change in elevation between the red and grey pins has increased

What have we achieved with all this?

Let’s review:

  • The premise is that we are shooting sunrise on some mountain peaks high above an extensive plain
  • 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 grey pin in a likely-looking location, Lock the elevation at the horizon to the grey pin 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 grey pin next
  • With a little trial and error, we can get a decent estimate of where the visible horizon will lie
  • You can see by now that 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


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

  • The distance to the horizon will vary depending on which direction you look. Therefore, it’s important to establish the horizon in the direction from which the light appears or in which you plan to shoot. (In the example above, the distance to the horizon in the east is very different from the distance to the horizon in 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 horizon overlay locations than the flatter terrain 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 the horizon by clicking the horizon lock again – this leaves the grey pin free but the elevation at the horizon remains set
  • To clear the elevation at the horizon, click on the value in the horizon text box, delete the value and then hit return.

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.

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