Using TPE Desktop Web App, Part 2: Beyond the Basics

· |

This is the second in a series of tutorials on The Photographer's Ephemeris desktop web app.

We covered the basics of using the program in Part 1. In Part 2, we’ll cover the guts of TPE: the timeline and the chart. We’ll also discuss screen sizing, take a brief look at twilight and see how TPE sets out information on shadows.

This tutorial is based on version 0.9.7. Click on any screenshot for a full-size expanded view.

Choosing a location

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

  • Click into the Search text box (above the map), type “Mount Sneffels, Colorado, USA” (just typing “Sneffels” takes you to a place in Iceland, after which Mt. Sneffels was originally named), and click search or hit return to perform the search.
  • You should see the primary map marker (the red pin) at the summit of Mount Sneffels, one of Colorado's finest fourteeners (summit over 14,000ft)
  • If you want to follow along, set the date to 3 August 2009. One nice thing about TPE is that you can look backward as well as forward in time: the information is correct for the date and red pin position you have selected.

To change the date in the calendar: click on the calendar button, click the month name at the top, click the year number at the top. Choose the year (2009), choose the month (August), choose the day.

Your screen should look something like this:

TPE Web App Tutorial Image 1

  1. The search box shows the search term you entered
  2. The red pin is positioned over the closest matching location (the summit of Mount Sneffels in this case)
  3. The date is set to 3 August 2009

An introduction to the events timeline

Let’s move the red pin to the southwest, along the trail above the highest of the three Blue Lakes. (Why there? Why then? I was there and took some photos! You can see some of the images here)

You can centre the map on the red pin by holding down Shift and clicking the centre pin button, top right on the map, or by using the keyboard shortcut: Shift C

Now, look at the events timeline below the map.

You can see from the timeline that the first “event” for 3 August 2009 is moonset at 03:53. The time of moonrise and set vary significantly through the month: certain days will not even have a moonrise or set event. The other information in the moonset box is the azimuth of the moonset, its phase and its percentage visibility. If this were a new moon or a full moon there would be additional information in the box.

The next three events are the three standard twilights: astronomical, nautical and civil. After that come sunrise, moonrise and sunset and then the three twilights again.

TPE Web App Tutorial Image 2

  1. The events timeline for the date 3 August 2009
  2. The centre red pin button
  3. Moonset, at 03:53, is the first event of the day

Size is everything

At this point I want to draw your attention to screen sizing. I am using a small screen for this tutorial and not all of the event information in the timeline is viewable at this size. If you have a small screen just click and drag the timeline left and right with a mouse (or use a two-finger scroll with a trackpad) to view all of the events in the timeline for the day.

When panning the map in search of a location, it is sometimes desirable to have a bigger map area. There are handy features that allow you to achieve this.

Look at the two blue buttons to the far right, above the map. The left one toggles the events timeline on and off and the right toggles full screen mode which hides or shows the chart AND timeline and widens the display to occupy the full width of the monitor. In full screen mode the time slider and legend remain in view. The keyboard shortcuts for changing the viewable map area are: “T” for toggling the timeline on and off and “F” for toggling full screen mode on and off.

If you like using the full width of your display, you can permanently set widescreen mode in the settings menu.

TPE Web App Tutorial Image 3

  1. Select widescreen mode from the settings menu
  2. Event timeline toggle on and off
  3. Full screen mode toggle on and off
  4. If you have a small screen, scroll left and right on the timeline to view all of the day’s events

An introduction to the chart and time slider

If you have been toggling full screen on and off, make sure the chart and timeline are in view at the bottom of the screen.

Look at the chart, it displays the altitude of the sun and moon over the course of the selected day: the sun in orange, the moon in blue.

The time slider, below the chart, allows you to change the time of day.

Scrub the time slider left and right. The legend moves with the slider and shows the time selected, and the azimuth and altitude information for the sun (orange) and the moon (blue). Here it is important to note that since both azimuth and altitude are expressed in degrees, we add + or – to the altitudes to distinguish them from the azimuths.

The sun azimuth line shows the bearing of the sun from the red pin at 19:06, the selected time on the legend. The azimuth line for the sun is orange and for the moon in blue.

Scrub the time slider again and watch the lines on the map: notice that thin sun and moon azimuth lines move around between the thicker rise and set lines. If the sun or moon lies above the horizon, these azimuth lines show the bearing for the selected time.

You can make finer adjustments with the time slider. Click then release the time slider (it highlights blue), now use your left and right keyboard cursor keys to move the time slider in one-minute increments.

Clicking on an event in the timeline sets the time slider to that time. Try it now, click on the Sunrise box in the timeline. If your date is still set to 3 August 2009, the time slider jumps to 06:17. You can also move left and right through each event of the days timeline by using the keyboard shortcuts: “,” (comma) and “.” (full stop). In the image below I have clicked on the moonrise event for the day.

TPE Web App Tutorial Image 4

  1. The chart
  2. The time slider
  3. The legend
  4. The sun azimuth line
  5. The selected event

Twilight

TPE’s glossary contains definitions of various concepts and terms used in TPE. The definitions of the three twilight states are discussed in more detail there, but in summation are as follows: astronomical twilight occurs when the sun lies between 18° and 12° below the horizon; nautical when the sun lies between 12° and 6°; and civil when the sun lies between 6° and 0°.

The light of twilight is variously described as more “even”, muted or blue than full sunshine. This lower dynamic range suits the camera and can make for great landscape photographs. The blue “hour” is a myth, the actual length of twilight varies significantly by season and by latitude: it is short in the tropics but long in the polar summer.

Twilight can affect photography in a number of important ways. At temperate latitudes, such as here in Colorado (40°N) late Nautical and early Civil twilight offers up more intense sky colours than late Civil twilight. And alpenglow will typically last until 10-15 minutes before sunrise – roughly mid-way through the typical Civil twilight period.

It is important to note that though the sun may not have yet risen (or may have already set), twilight is still directional, hence the inclusion of the twilight azimuth information, along with twilight start or end time, in the timeline. The azimuth corresponds to the brightest point on the horizon.

Let’s look at using the twilight information in the timeline for a practical example from our Blue Lakes location on 3 August 2009.

Imagine you wanted to do some night photography of the Blue Lakes and the surrounding mountains with a clear, starry sky overhead. When would be a suitable time to shoot that during the night of 3 August 2009?

TPE Web App Tutorial Image 5

  1. The glossary
  2. Twilight information
  3. Moonset
  4. Astronomical twilight begins

If you’re looking to shoot a clear, starry sky, you probably want it to be truly dark i.e. after the moon has set and before twilight. Using the information in the timeline, 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 not be visible in the sky.

Shadows

We all know that as the sun gets lower in the sky, shadows lengthen towards infinity. The moon casts shadows too, especially at full moon. In TPE, shadows are represented by the +6° shadow circle. (NOTE: TPE limits the length of its shadow and azimuth lines to 200 miles.)

You should still have the date set at 3 August 2009, click the sunrise event in the timeline for that day. Now view the shadow circle by holding down the shift key (on Mac you can lock the shadow circle by using the caps lock). A dark circle appears within the map, centred on the red pin.

Now slowly scrub the time slider forward from sunrise. Two things immediately happen: a) the circle turns orange and b) the sun azimuth line now has a thinner line extending through the red pin position. This thinner line may well be overlayed with a dark brown line depending on how far you just moved the time slider (more on this later).

The circle turns orange because the sun is now above +0°. Keep scrubbing the slider to the right and the orange circle will turn black again when the sun reaches +6°. A sun altitude of less than +6° corresponds to golden light (due to atmospheric scattering) and a great time for photography. If you scrub all the way towards sunset, the same thing happens at the end of the day. In the image below you can see the sun altitude information in the legend shows that the sun is below +6° for the time selected.

The sun and moon azimuth extension lines have two purposes. First they are useful for finding the time of alignment of the sun or moon with a particular object or landscape feature: scrubbing the time slider and observing the azimuth extension lines allow you determine alignment visually. Many “henge” shots have been planned using this method.

The second purpose of the azimuth extension line is to show shadow length. Shadow length is represented by the thicker dark line that overlays the azimuth extension line (brown for sun, dark blue for moon). While holding down shift, scrub the slider through the day to see the shadow length change. When the sun shadow falls outside the circle this corresponds to the sun’s being below +6° and the circle turns orange.

TPE Web App Tutorial Image 6

  1. The +6° shadow circle is orange when the sun is between +0° and +6°
  2. The sun azimuth extension line
  3. The sun shadow line over the sun azimuth extension line
  4. The sun altitude information in the legend

Next time we’ll get a little bit geeky and look at some more advanced TPE features.

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.