How to use it
- The map opens at the present moment. Pick another date and time in UTC if you like.
- Press Play to run through the day; choose the step size with the chips.
- The yellow pin is where the sun is straight overhead; the bands are the twilights.
5 practical uses
- See who is awake before a call across time zones.
- Show the seasons and the terminator to a class.
- Track daylight for a flight across the world.
- Check where it is dark for radio, astronomy or satellite work.
- Follow the sun through the day on a wall screen.
The maths behind it
The sub-solar point is at the sun's declination δ and at the longitude where the sun is on the meridian, from the Greenwich sidereal time. On the terminator the sun's altitude is zero, so for each longitude the latitude satisfies sin φ sin δ + cos φ cos δ cos H = 0, that is tan φ = −cos H / tan δ, where H is the hour angle. The twilight bands use altitudes of −6°, −12° and −18° in the same equation.
What you see
The edge of the shadow is the terminator. It is a great circle, and on this map it appears as a curve because the map is flat; only at the equinoxes does it run straight from pole to pole. In June it wraps around the North Pole, leaving the Arctic in daylight all night; in December it wraps around the south. The sub-solar point moves between the tropics through the year and around the world once a day, at 15° of longitude an hour.
The shape is computed from the sun's declination and hour angle for the moment chosen, to a fraction of a degree.