Day and Night World Map

The subsolar point is the location on Earth's surface where the Sun is directly overhead — at the zenith — at a specific moment in time. At this point, the Sun's rays hit Earth at a 90° angle. The subsolar point moves westward as Earth rotates and shifts between the Tropics of Cancer and Capricorn over the course of a year.

UTC (Coordinated Universal Time) is the global time standard that doesn't change with seasons or time zones. Using UTC ensures that solar position calculations are consistent and unambiguous regardless of where you are located. Your local time is simply UTC offset by your time zone difference.

Solar declination is the angle between the Sun's rays and Earth's equatorial plane. It ranges from approximately +23.44° at the June solstice to −23.44° at the December solstice. Declination determines the latitude of the subsolar point and directly affects how much daylight different latitudes receive.

The equation of time is the difference in minutes between true solar time and mean solar time (clock time). It arises because Earth's orbit is slightly elliptical and Earth's axis is tilted relative to its orbital plane. This difference can range from about −16 minutes to +14 minutes throughout the year.

These are three gradations of twilight based on how far the Sun is below the horizon. Civil twilight occurs when the Sun is 0°–6° below the horizon — enough light for most outdoor activities. Nautical twilight is 6°–12° below, when the horizon is still visible at sea. Astronomical twilight is 12°–18° below, after which the sky is considered fully dark for observing.

At the equator, the Sun rises and sets almost perpendicular to the horizon throughout the year. Because the equator is equidistant from both poles, it receives roughly equal day and night year-round, with day length varying only slightly (about 10–20 minutes) due to atmospheric refraction and the apparent size of the Sun.

The Sun is directly overhead at the Tropic of Cancer (23.44°N) on approximately June 21 — the June solstice — and at the Tropic of Capricorn (23.44°S) on approximately December 21 — the December solstice. Locations between the two tropics experience the Sun directly overhead twice per year.

The subsolar longitude moves westward at about 15° per hour as Earth rotates. At 12:00 UTC (solar noon at Greenwich), the subsolar longitude is approximately 0° (the Prime Meridian). At 18:00 UTC it would be near −90°, and so on. This is why every 15° of longitude corresponds to one hour of time difference.