Astronomical Terminology

The main astronomical terminology used on the site in relation to earth-bound activities!

Sunrise, sunset and twilight are all defined as if there are ideal meteorological conditions and an ideal horizon. In practice poor weather conditions will often obscure these events and any features such as hills and mountains, will also impact any observations.

Golden Hour

In photography, this is the time of the day where the sun has just risen and appears warmer (redder and softer). This effect is created by the sun's rays being filtered by the earth's atmosphere, or more correctly as the bluer light is scattered more.

Sunrise and sunset

Sunrise

The instance that the edge of the sun's disk rises above the ideal horizon.

Sunset

The instance that the final part of the sun's disk falls above above the horizon.

Twilight

Time of the day where the sun helps to illuminate the sky from below the horizon, excluding the illumination from the moon.

Civil twilight

It is generally light enough to do activities outdoors without the aid of artificial light.

Most of the sunlight that illuminates things is that of the bluer end of the visible spectrum that gets scattered more easily. This is why much of this twilight period is known in photography as the Blue Hour. The Magic Hour is the period closest to the sunrise where it is bright and that there are no sharp shadows.

This is defined where the sun is less than 6° below an ideal horizon.

Nautical Twilight

Artificial light is normally required for most outdoor activities. Under good atmospheric conditions, vague outlines of ground objects are barely distinguishable. The horizon is often discernible, although usually not at sea. Stars required for navigational purposes are still clearly visible, and this is where it got its name.

This is defined where the depression angle of the sun to the horizon is between 6° and 12°.

Nautical Twilight

The illumination from the scattered light from the Sun is less than that the constant natural light sources in the sky such as starlight.

This is defined where the depression angle of the sun to the horizon is between 12° and 18°.

Night

Time of the day where none of the sun's rays help to illuminate the sky other than those that may be reflected off the moon.

Sun Transit Time

This is the moment that the sun crosses the observer's meridian (the imaginary line running from true north to true south. For most observers this is where the sun is at its highest point in the sky midway though the day. For those at high altitudes, there may be multiple transits between sunrise and sunset. Also known as local solar (sundial) noon.

Moonrise and moonset

Moonrise

The moment the moon's disk is visible in perfect weather.

Moonset

The final moment before the moon's disk vanishes below the surface.

Moon phases

The phases are created by the difference in longitudes of the Moon and Sun that alter the moon's illumination. While the phases of new, quarter and full moons are instantaneous moments in time, each phase is usually given an equal amount of time within this cycle that is just under four days. While the moon only takes 27.3 days to complete its orbit around the Earth, there are approx 29.5 days in each moon phase cycle. The difference is due to the fact that the earth is also rotating around the sun, so this extends the time of the phase as the sun has a different angle to the earth.

Equinoxes and Solstices

Equinox

Day and night are of equal length that happens once in Spring and once in Autumn.

Solstice

When the day (summer) or night (winter) is at its longest.