Space and Earth Calculators

Space and earth calculators apply the equations of positional astronomy to work out where and when celestial bodies appear in the sky for a given place and date. Their results depend on the observer's latitude and longitude, the calendar date, and the Earth's rotation and orbit, so the same instant produces different sunrise times in different cities.

Sunrise and sunset calculators determine the moments the Sun's upper edge crosses the horizon, accounting for atmospheric refraction, which raises the Sun's apparent position near the horizon. Twilight calculators extend this to the periods before sunrise and after sunset when scattered sunlight still lights the sky, conventionally divided into civil, nautical and astronomical twilight according to how far the Sun lies below the horizon. Moonrise and moonset tools perform the same geometry for the Moon, whose faster apparent motion makes its rising time shift markedly from night to night.

Moon-phase calculators track the changing illuminated fraction of the Moon over its roughly 29.5-day synodic cycle, from new moon through first quarter, full moon and last quarter. Eclipse calculators predict solar and lunar eclipses, which occur only when the Sun, Earth and Moon align closely enough near the points where the Moon's orbit crosses the plane of Earth's orbit. Tools that give the positions of the Sun and Moon report altitude and azimuth, the angle above the horizon and the compass direction, which are useful for navigation, photography and observing.

Frequently asked questions

Why do sunrise times differ from one location to another?

Sunrise depends on latitude and longitude relative to the Sun's position, so places at different positions on Earth see the Sun cross the horizon at different clock times.

What are the three kinds of twilight?

Civil, nautical and astronomical twilight, defined by how many degrees the Sun is below the horizon; astronomical twilight ends when the sky is fully dark.

Why don't eclipses happen every month?

The Moon's orbit is tilted relative to Earth's orbit, so the three bodies line up closely enough for an eclipse only at certain times, not at every new or full moon.