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The astronomical unit is typically used for stellar system scale distances, such as the size of a protostellar disk or the heliocentric distance of an asteroid, whereas other units are used for other distances in astronomy. The astronomical unit is too small to be convenient for interstellar distances, where the parsec and light-year are
The astronomical unit of length is now defined as exactly 149 597 870 700 meters. [4] It is approximately equal to the mean Earth–Sun distance. It was formerly defined as that length for which the Gaussian gravitational constant (k) takes the value 0.017 202 098 95 when the units of measurement are the astronomical units of length, mass and ...
The calculated stellar distance will be in the same measurement unit as used in Distance earth-sun (e.g. if Distance earth-sun = 1 au, unit for Distance star is in astronomical units; if Distance earth-sun = 1.5813 × 10 −5 ly, unit for Distance star is in light-years).
Distance moduli are most commonly used when expressing the distance to other galaxies in the relatively nearby universe.For example, the Large Magellanic Cloud (LMC) is at a distance modulus of 18.5, [2] the Andromeda Galaxy's distance modulus is 24.4, [3] and the galaxy NGC 4548 in the Virgo Cluster has a DM of 31.0. [4]
The light-year unit appeared in 1851 in a German popular astronomical article by Otto Ule. [24] Ule explained the oddity of a distance unit name ending in "year" by comparing it to a walking hour (Wegstunde). A contemporary German popular astronomical book also noticed that light-year is an odd name. [25]
Lunar distance (LD), the distance from the centre of Earth to the centre of the Moon, is a unit of measure in astronomy. The lunar distance is approximately 384,400 km (238,900 mi), or 1.28 light-seconds; this is roughly 30 times Earth's diameter. A little less than 400 lunar distances make up an astronomical unit.
The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A direct distance measurement of an astronomical object is possible only for those objects that are "close enough" (within about a thousand parsecs ) to Earth.
Average distance between the center of Earth and the center of the Moon. astronomical unit au. Defined as 149 597 870 700 m. [16] Approximately the distance between the Earth and Sun. light-year ly ≈ 9 460 730 472 580.8 km. The distance that light travels in a vacuum in one Julian year. [17] parsec pc ≈ 30 856 775 814 671.9 km or about 3. ...