Astronomical unit

The grey line indicates the Earth–Sun distance, which on average is about 1 astronomical unit.
Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called "black drop effect") and the rarity of observations.
The astronomical unit is used as the baseline of the triangle to measure stellar parallaxes (distances in the image are not to scale)

Unit of length, roughly the distance from Earth to the Sun and equal to 150 e6km or 8.3 light minutes.

- Astronomical unit

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Unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 1 pc or 1 pc (au), i.e. 30.9 e12km.

A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale)
Stellar parallax motion from annual parallax
As observed by the Hubble Space Telescope, the astrophysical jet erupting from the active galactic nucleus of M87 subtends 20 arcsecond and is thought to be 1.5 kpc long (the jet is somewhat foreshortened from Earth's perspective).

The two dimensions on which this triangle is based are its shorter leg, of length one astronomical unit (the average Earth-Sun distance), and the subtended angle of the vertex opposite that leg, measuring one arcsecond.


Farthest or nearest point in the orbit of a planetary body about its primary body.

The apsides refer to the farthest (1) and nearest (2) points reached by an orbiting planetary body (1 and 2) with respect to a primary, or host, body (3).
For example, the Moon's two apsides are the farthest point, apogee, and the nearest point, perigee, of its orbit around the host Earth. The Earth's two apsides are the farthest point, aphelion, and the nearest point, perihelion, of its orbit around the host Sun. The terms aphelion and perihelion apply in the same way to the orbits of Jupiter and the other planets, the comets, and the asteroids of the Solar System.
The two-body system of interacting elliptic orbits: The smaller, satellite body (blue) orbits the primary body (yellow); both are in elliptic orbits around their common center of mass (or barycenter), (red +).
∗Periapsis and apoapsis as distances: The smallest and largest distances between the orbiter and its host body.
Keplerian orbital elements: point F, the nearest point of approach of an orbiting body, is the pericenter (also periapsis) of an orbit; point H, the farthest point of the orbiting body, is the apocenter (also apoapsis) of the orbit; and the red line between them is the line of apsides.
Diagram of a body's direct orbit around the Sun with its nearest (perihelion) and farthest (aphelion) points.
The perihelion (green) and aphelion (orange) points of the inner planets of the Solar System
The perihelion (green) and aphelion (orange) points of the outer planets of the Solar System

At perihelion, the Earth's center is about 0.983 astronomical units (AU) or 147098070 km from the Sun's center.

Solar mass

Standard unit of mass in astronomy, equal to approximately 2 kg. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes.

The Sun contains more than 99% of the mass of the Solar System. Bodies lighter than Saturn are not visible at this scale.

Based on the length of the year, the distance from Earth to the Sun (an astronomical unit or AU), and the gravitational constant (

Cosmic distance ladder

Succession of methods by which astronomers determine the distances to celestial objects.

The Astronomer shows the use of parallax to measure distance. It is made from parts of the Yale–Columbia Refractor telescope (1924) damaged when the 2003 Canberra bushfires burned out the Mount Stromlo Observatory; at Questacon, Canberra.
Stellar parallax motion from annual parallax. Half the apex angle is the parallax angle.
Parallax measurements may be an important clue to understanding three of the universe's most elusive components: dark matter, dark energy and neutrinos.
Hubble precision stellar distance measurement has been extended 10 times further into the Milky Way.
SN 1994D (bright spot on the lower left) in the NGC 4526 galaxy. Image by NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team
Galaxy cluster

Direct distance measurements are based upon the astronomical unit (AU), which is defined as the mean distance between the Earth and the Sun.


Large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers , or 5.88 trillion miles (5.88 mi).

Map showing the stars that lie within 12.5 light-years of the Sun

The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry; it is the distance at which one astronomical unit subtends an angle of one second of arc.


Displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

A simplified illustration of the parallax of an object against a distant background due to a perspective shift. When viewed from "Viewpoint A", the object appears to be in front of the blue square. When the viewpoint is changed to "Viewpoint B", the object appears to have moved in front of the red square.
This animation is an example of parallax. As the viewpoint moves side to side, the objects in the distance appear to move more slowly than the objects close to the camera. In this case, the white cube in front appears to move faster than the green cube in the middle of the far background.
In this photograph, the Sun is visible above the top of the streetlight. In the reflection on the water, the Sun appears in line with the streetlight because the virtual image is formed from a different viewing position.
Parallax is an angle subtended by a line on a point. In the upper diagram, the Earth in its orbit sweeps the parallax angle subtended on the Sun. The lower diagram shows an equal angle swept by the Sun in a geostatic model. A similar diagram can be drawn for a star except that the angle of parallax would be minuscule.
Hubble Space Telescope – Spatial scanning precisely measures distances up to 10,000 light-years away (10 April 2014).
Stellar parallax motion
Diagram of daily lunar parallax
Example of lunar parallax: Occultation of Pleiades by the Moon
Measuring Venus transit times to determine solar parallax
The correct line of sight needs to be used to avoid parallax error.
Contax III rangefinder camera with macro photography setting. Because the viewfinder is on top of the lens and of the close proximity of the subject, goggles are fitted in front of the rangefinder and a dedicated viewfinder installed to compensate for parallax.
Failed panoramic image due to the parallax, since axis of rotation of tripod is not same of focal point.
Simple animation demonstrating the effects of parallax compensation in telescopic sights, as the eye moves relative to the sight.
Parallax theory for finding naval distances

The diurnal parallax has been used by John Flamsteed in 1672 to measure the distance to Mars at its opposition and through that to estimate the astronomical unit and the size of the Solar System.

Lunar distance (astronomy)

Moon's average distance to Earth.

Distance between the Earth and Moon (mean value) – sizes and distance to scale.
Photo of Earth and Moon, taken by the OSIRIS-REx probe
The Moon's distance from the Earth and Moon phases in 2014. Moon phases: 0 (1) – new moon, 0.25 – first quarter, 0.5 – full moon, 0.75 – last quarter.
Variation of the distance between the centers of the Moon and the Earth over 700 days.
Lunar Laser Ranging Experiment from the Apollo 11 mission

A little less than 400 lunar distances make up an astronomical unit.

Minute and second of arc

Unit of angular measurement equal to 1⁄60 of one degree.

An illustration of the size of an arcminute (not to scale). A standard association football (soccer) ball (with a diameter of 22 cm) subtends an angle of 1 arcminute at a distance of approximately 756 m.
Comparison of angular diameter of the Sun, Moon, planets and the International Space Station. True represent­ation of the sizes is achieved when the image is viewed at a distance of 103 times the width of the "Moon: max." circle. For example, if the "Moon: max." circle is 10 cm wide on a computer display, viewing it from 10.3 m away will show true representation of the sizes.
Example ballistic table for a given 7.62×51mm NATO load. Bullet drop and wind drift are shown both in mrad and minute of angle.
Comparison of milliradian (mrad) and minute of arc (MOA).

an object of diameter 725.27 km at a distance of one astronomical unit,

Unit of length

A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length.

A ruler, depicting two customary units of length, the centimetre and the inch
Diagram of English length units (all units of length defined by Appendix C of the NIST Handbook) and their integer relations to each other.
Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service

astronomical unit au. Defined as 149,597,870,700 m. Approximately the distance between the Earth and Sun.

Earth's orbit

Earth at seasonal points in its orbit (not to scale)
Earth orbit (yellow) compared to a circle (gray)
Heliocentric Solar System
Heliocentrism (lower panel) in comparison to the geocentric model (upper panel), not to scale

Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere.