Declination

In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that help us find a point on the celestial sphere using the equatorial coordinate system. The other angle is called the hour angle. Declination measures how far north or south a point is from the celestial equator, which is an imaginary line in the sky that matches Earth's equator.

The word declination comes from Latin, declinatio, meaning "a bending away" or "a bending down." It shares its roots with words like incline (to bend forward) and recline (to bend backward). This idea of bending helps us understand how points in the sky are positioned above or below the celestial equator.

Long ago, in the 18th and 19th centuries, some astronomy books used a different way to talk about declination. They called it North Pole Distance (N.P.D.). This was just another way to measure the distance from the north celestial pole.

Explanation

Main article: Equatorial coordinate system

Declination in astronomy is like latitude on Earth, but it is used for points in the sky on the celestial sphere. Points above the celestial equator have positive declination, and points below have negative declination. Declination is usually measured in degrees, minutes, and seconds of sexagesimal measure.

For example:

• An object on the celestial equator has a declination of 0°
• The north celestial pole has a declination of +90°
• The south celestial pole has a declination of −90°

The sign is always included, whether the number is positive or negative.

Effects of precession

Main article: Axial precession

The Earth's axis slowly moves in a westward direction, making a full circle about every 26,000 years. This movement is called precession. Because of this, the positions of stars and other objects in the sky change slowly over time.

Astronomers need to say what year they measured these positions, called an epoch.

The standard epoch used today is J2000.0, which means January 1, 2000, at noon. Before this, astronomers used different epochs such as Besselian Epochs like B1875.0, B1900.0, and B1950.0.

Stars

Stars look like they stay in the same place because they are very far away. But their positions, including something called declination, change slowly over time. This happens because of how the Earth moves and how the stars move.

From places in the Northern Hemisphere, stars with high declination values seem to move in circles around the North Pole. They never go below the horizon. These are called circumpolar stars. In the Southern Hemisphere, stars with very low declination values also never go below the horizon. For example, the pole star has a declination close to +90°, so it is always visible in the Northern Hemisphere.

Some stars are only visible during certain times of the year. These are called non-circumpolar stars.

Sun

Main article: Position of the Sun

The Sun's declination changes with the seasons. From places far north or south, like the arctic or antarctic areas, the Sun can stay above the horizon all night around the time of local summer solstice. This is called the midnight sun. In contrast, during the local winter solstice, the Sun may stay below the horizon all day, creating what is called polar night.

Relation to latitude

When an object is right above us, its declination is almost the same as our latitude. Latitude shows how far north or south a place is on Earth. But there are tiny differences because of how we measure these positions.

In places like the United States, these differences are usually just a few arcseconds — a very small part of a degree. The exact point "overhead" can also be slightly different because of the shape of the Earth. Special books called almanacs give us the declination measured from the Earth's center.