This is the second in a series of tutorials on Photo Ephemeris Web.
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 2.0.32.
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 the Search button above the map to open the search form.
- Type “Mount Sneffels, Colorado, USA”
- Click search or hit return to perform the search. (If you're a PRO subscriber, you'll see results as you type.)
- Click Go on the first displayed result
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 2021. (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 date selector, choose the date from the calendar, or just type it into the text field and hit enter. Your screen should look something like this:
- Primary pin is at the summit of Mount Sneffels
- Date is set to Tues Aug 3 2021
An introduction to the 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 a couple of the of the images here and here.)
You can center 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 Timeline below the map.
You can see from the timeline that the first “event” for 3 August 2021 is moonset at 01:48.
The next three events are the three standard twilights: astronomical (12-18° below the horizon), nautical (6-12°) and civil (6-0°). After that comes sunrise, golden hour, moon transit, then sun transit, and so on. We define golden hour as the time between sunrise and when the sun climbs to +6° above the horizon (and the time between +6° and sunset). Transit is the time when the sun or moon crosses the local meridian (due north or due south) - typically this is also the instant of highest altitude (elevation angle) above the horizon, known as culmination.
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.
- Timeline for 3 August 2021
- Recenter the red pin on the map
- Moonrise at 01:48 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 some controls that allow you to achieve this.
Look at the four blue buttons to the right above the map. They control what is displayed on screen. From left to right:
- Show map, timeline and altitude chart
- Hide timeline
- Hide altitude chart
- Hide both timeline and altitude chart
You can cycle through these options using “T” key on your keyboard.
In addition, there is a Full screen button at the top right of the map which expands the viewable map area by hiding the page header and footer.
- Display controls
- Full screen mode toggle on and off
- If you have a narrow window, scroll left and right on the timeline to view all of the day’s events
An introduction to the chart and time slider
For this section of the tutorial, ensure both the chart and timeline are visible.
Look at the chart: it displays the altitude (elevation angle) 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, plus the azimuth and altitude information for the sun (orange) and the moon (blue). It is important to note that since both azimuth and altitude are expressed in degrees, we add + or – to altitude values to distinguish them from azimuths.
The sun azimuth line shows the bearing of the sun from the red pin at 16:50 (4:50pm), the selected time on the legend. The azimuth line for the sun is orange (the moon appears in blue, but this is the time of moonset).
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 you would look along from the red pin position at the selected time.
You can make finer adjustments with the time slider. Click the time slider handle: now use the left and right cursor keys on your keyboard to move the time slider back and forth in ten second increments.
Clicking on an event in the timeline sets the time slider to that time. Try it now, click on the Moonset panel in the timeline. If your date is still set to 3 August 2021, the time slider jumps to 16:50. You can also move left and right through each event of the day's timeline by using the keyboard shortcuts: “,” (comma) and “.” (full stop). In the image below I have clicked on the moonset event for the day. If the selected time of day matches a timeline event, the event title is shown in the chart legend:
- Altitude chart
- Time slider
- Chart legend
- Sun azimuth line
- Selected event
The definitions of the three twilight states 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°. You can find more details on these terms on Wikipedia.
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. (Read more: Magical Gold and Blue: Confusion in the Twilight Zone)
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 20 August 2021.
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 20 August 2021?
- Civil twilight starts
- 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 astronomical twilight begins. 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 04:07 am, but astronomical twilight begins at 04:55 am. It’s likely that the best time is somewhere between 04:15 and 04:40am. Once astronomical twilight begins, objects such as the Milky Way will become less visible in the sky, disappearing altogether as nautical twilight progresses.
We all know that as the sun gets lower in the sky, shadows lengthen towards infinity. The moon casts shadows too, especially noticeable 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.)
Set the date to 03 August 2021, then click the sunrise event in the timeline. Now view the shadow circle by holding down the handle of the time slider. 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 overlaid 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). 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.
- "+6° shadow circle" is orange when the sun is between +0° and +6°
- Sun azimuth extension line
- Sun shadow line over the sun azimuth extension line
- Sun altitude shown in the chart legend
Next time we’ll get a little bit geeky and look at some more advanced TPE features: Using TPE Desktop Web App, Part 3: Geodetics