Tuesday Aug 11, 2009 at 00:07

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:

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:

Viewing twilight information

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):

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.

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?

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

Posted in TPE

Stephen · Tuesday, August 11, 2009, 00:07 · Permalink

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