Scientific topics Astronomical definitions

Sunrise and sunset

Sunrise

Sunrise is defined as the instant in the morning under ideal meteorological conditions, with standard refraction of the Sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.

Sunset

Sunset is defined as the instant in the evening under ideal meteorological conditions, with standard refraction of the Sun's rays, when the upper edge of the sun's disk is coincident with an ideal horizon.

Civil twilight

Beginning of morning civil twilight

Defined as the instant in the morning, when the centre of the Sun is at a depression angle of six degrees (6°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, the illumination is such that large objects may be seen but no detail is discernible. The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined.

Ending of evening civil twilight

Defined as the instant in the evening, when the centre of the Sun is at a depression angle of six degrees (6°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, the illumination is such that large objects may be seen but no detail is discernible. The brightest stars and planets can be seen and for navigation purposes at sea, the sea horizon is clearly defined.

Nautical twilight

Beginning of morning nautical twilight

Is defined as the instant in the morning, when the centre of the Sun is at a depression angle of twelve degrees (12°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, it is dark for normal practical purposes. For navigation purposes at sea, the sea horizon is not normally visible.

Ending of evening nautical twilight

Is defined as the instant in the evening, when the centre of the Sun is at a depression angle of twelve degrees (12°) below an ideal horizon. At this time in the absence of moonlight, artificial lighting or adverse atmospheric conditions, it is dark for normal practical purposes. For navigation purposes at sea, the sea horizon is not normally visible.

Astronomical twilight

Beginning of morning astronomical twilight

Is defined as the instant in the morning, when the centre of the Sun is at a depression angle of eighteen degrees (18°) below an ideal horizon. At this time the illumination due to scattered light from the Sun is less than that from starlight and other natural light sources in the sky.

Ending of evening astronomical twilight

Is defined as the instant in the evening, when the centre of the Sun is at a depression angle of eighteen degrees (18°) below an ideal horizon. At this time the illumination due to scattered light from the Sun is less than that from starlight and other natural light sources in the sky.

Sun transit time

The transit time

The transit time of a celestial body refers to the instant that its center crosses an imaginary line in the sky - the observer's meridian - running from north to south. For observers in low to middle latitudes, transit is approximately midway between rise and set, and represents the time at which the body is highest in the sky on any given day. At high latitudes, neither of these statements may be true - for example, there may be several transits between rise and set. The transit of the Sun is local solar (sundial) noon.

Moonrise and moonset

Moonrise

Moonrise is defined as the instant when, in the eastern sky, under ideal meteorological conditions, with standard refraction of the Moon's rays, the upper edge of the Moon's disk is coincident with an ideal horizon.

Moonset

Moonset is defined as the instant when, in the western sky, under ideal meteorological conditions, with standard refraction of the Moon's rays, the upper edge of the Moon's disk is coincident with an ideal horizon.

Moon phases

As the relative position of the Sun, Moon and Earth changes, differing proportions of the Moon's visible surface are illuminated by the Sun. The phases of the Moon are specific instances in this process.

New moon

A new Moon occurs when the apparent longitudes of the Moon and Sun differ by 0°. At this time, the Moon does not appear to be illuminated.

First quarter

Occurs when the apparent longitudes of the Moon and Sun differ by 90°. At this time 50 per cent of the Moon's visible surface is illuminated.

Full moon

Occurs when the apparent longitudes of the Moon and Sun differ by 180°. At this time 100 per cent of the Moon's visible surface is illuminated.

Last quarter

Occurs when the apparent longitudes of the Moon and Sun differ by 270°. At this time 50 per cent of the Moon's visible surface is illuminated.

Rise and set of five planets

The planets which are generally visible to the naked eye are: Mercury, Venus, Mars, Jupiter and Saturn.

The rise and set times for these planets is the instant when, under ideal meteorological conditions, with standard refraction of their rays, the planet is coincident with an ideal horizon.

Ideal Horizon

An ideal horizon exists when the surface forming the horizon is at a right angle to the vertical line passing through the observer's position on the Earth. If the terrain surrounding the observer was flat and all at the same height above sea level, the horizon seen by the observer standing on the Earth would approximate the ideal horizon.

Zenith Distance

The zenith distance is a vertical angle measured from directly overhead, down to the required point. An ideal horizon has a zenith distance of 90 degrees.

True Azimuth

True azimuth is the clockwise horizontal angle from true north to the object being sighted.

True north

True north is the direction towards the north pole along the meridian of longitude which passes through the observer's position on the Earth.

Vertical angle

The vertical angle is the angle measured in a vertical plane, from the horizon to the required point. Directly overhead would have a vertical angle of 90 degrees.

The perigee and apogee table

All dates and times are Universal Time (UTC) . To convert to local time add or subtract the difference between your time zone and UTC, remembering to include any additional offset due to summer time for dates when it is in effect. For each perigee and apogee the distance in kilometres between the centres of the Earth and Moon is given. Perigee and apogee distances are usually accurate to within a few kilometres compared to values calculated with the definitive ELP 2000-82 theory of the lunar orbit; the maximum error over the years 1977 through 2022 is 12km in perigee distance and 6km at apogee.

The closest perigee and most distant apogee of the year are marked with "++" if closer in time to full Moon or "--" if closer to new Moon. Other close-to-maximum apogees and perigees are flagged with a single character, again indicating the nearer phase. Following the flags is the interval between the moment of perigee or apogee and the closest new or full phase; extrema cluster on the shorter intervals, with a smaller bias toward months surrounding the Earth's perihelion in early January. "F" indicates the perigee or apogee is closer to full Moon, and "N" that new Moon is closer. The sign indicates whether the perigee or apogee is before ("-") or after ("+") the indicated phase, followed by the interval in days and hours. Scan for plus signs to find "photo opportunities" where the Moon is full close to apogee and perigee.

Perihelion and Aphelion

Perihelion is the point in the orbit of a planet, asteroid, or comet at which it is closest to the Sun, while Aphelion is the point in a planet's orbit at which it is furthest from the sun, which is the opposite to Perihelion.

Equinoxes and Solstices

The equinoxes represents either of two times of the year when the Sun crosses the plane of the Earth's equator and day and night are of equal length, while the solstices is either of the two times of the year when the Sun is at its greatest distance from the celestial equator.