v3.14 of the Photo Ephemeris Web now displays the day/night terminator line and the sub-solar point on the map at lower zoom levels. Here’s a quick demo:
Day/Night Terminator
This line is an approximate representation of the set of points on the surface of the Earth that lie at the transition from day to night. It is useful for getting a quick sense of where the area of daylight (or night) lies at a glance. The movement of the terminator across the day and through the seasons can give a quick intuitive sense of how the sun is moving.
Additionally, ahead of the addition of more weather data to TPE, the terminator allows for a very quick assessment of weather conditions around the times of sunrise and sunset.
Note: the terminator line does not coincide exactly with local sunrise - sunrise etc. accounts refraction and the semi-diameter of the Sun, elevation above sea level and so on. However, it’s usually pretty close.
Sub-solar Point
In the latter part of the short video, you may notice a small black Sun icon moving on the map over the Pacific. That shows the point on Earth where the Sun lies directly overhead (i.e. altitude of 90 degrees, or a zenith angle of zero). The point moves westward with the rotation of the Earth around its own axis, and north/south between the tropics with the change of the seasons.
Let me know what you think - this is up now at app.photoephemeris.com