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A nephoscope emits a light ray, which strikes and reflects off the base of a targeted cloud. The distance to the cloud can be estimated using the delay between sending the light ray and receiving it back: distance = (speed of light × travel time) / 2
By combining many such measurements, a best fit value for the light time per unit distance could be obtained. For example, in 2009, the best estimate, as approved by the International Astronomical Union (IAU), was: [102] [103] light time for unit distance: t au = 499.004 783 836 (10) s, c = 0.002 003 988 804 10 (4) AU/s = 173.144 632 674 (3) AU/d.
It is described by the equation v = H 0 D, with H 0 the constant of proportionality—the Hubble constant—between the "proper distance" D to a galaxy (which can change over time, unlike the comoving distance) and its speed of separation v, i.e. the derivative of proper distance with respect to the cosmic time coordinate.
A proper motion of 1 arcsec per year 1 light-year away corresponds to a relative transverse speed of 1.45 km/s. Barnard's Star's transverse speed is 90 km/s and its radial velocity is 111 km/s (perpendicular (at a right, 90° angle), which gives a true or "space" motion of 142 km/s.
The comoving distance from an observer to a distant object (e.g. galaxy) can be computed by the following formula (derived using the Friedmann–Lemaître–Robertson–Walker metric): = ′ (′) where a(t′) is the scale factor, t e is the time of emission of the photons detected by the observer, t is the present time, and c is the speed of ...
Distance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe.They are often used to tie some observable quantity (such as the luminosity of a distant quasar, the redshift of a distant galaxy, or the angular size of the acoustic peaks in the cosmic microwave background (CMB) power spectrum) to another quantity that is ...
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] In 1868 an English journal labelled the ...
The particle horizon, also called the cosmological horizon, the comoving horizon, or the cosmic light horizon, is the maximum distance from which light from particles could have traveled to the observer in the age of the universe. It represents the boundary between the observable and the unobservable regions of the universe, so its distance at ...