Essential Astronomical Systems of Measurement

Speed of Light

The Speed of Light is around 186,000 miles/sec (299,792 km/sec), or around 671 million miles/hour (1.07 billion km/hour), meaning light travels a distance of about 5.88 trillion miles (9.5 trillion km) over a one year period. Here are some figures to put these speed and distances into perspective:

Nearest Star (Proxima Centauri): 4.2 ly
Brightest Star (Sirius): 8.6 ly
Milky Way Galaxy: 100,000 ly wide
Earth to Centre of Milky Way: 27,000 ly
Nearest Major Galaxy (Andromeda): 2 million ly
Brightest Stars of Orion: Rigel (864.3 ly) and Betelgeuse (642.5 ly)

Astronomical Unit

Astronomers use the distance between the Earth and the Sun as an astronomical measurement, with this length of space of about 93 million miles (150 million kilometres) referred to as an Astronomical Unit (AU). The distance between the Sun and the furthest planet Neptune, for instance, is around 30.1 AU (4.50 billion km), with the end of the solar system (heliopause) located around 100 AU from the Sun. In 2012, Voyager 1, travelling at 38,525 miles/hour (62,000 km/ hour), or 326,000,000 miles a year (3.5 AU per year), finally passed the heliopause after 35 years in space, although it has still a considerable distance to cover if the theoretical Oort cloud located 100,000 AU (1.87 light-years) away is taken to represent the end of our solar system. Nevertheless, Voyager 1 will not reach another star for almost 40,000 years, even at its incredible speed.


ParsecA Parsec is a unit of length used to measure the distances between objects located beyond our solar system, and is equal to 3.26 light-years (19 trillion miles/31 trillion km). Its name derives from the astronomical terms parallax (par) and arc second (sec), with a parsec marking the distance at which one astronomical unit is opposite and delimits an angle of one arcsecond. In the diagram shown, the letter d represents 1 parcec, while p is equal to an angular change in a star’s position of 1 arc-second.

For the largest astronomical structures in the Universe, astronomers resort to using kiloparsec (x 1 thousand parsecs), megaparsecs (x 1 million parsecs) and even gigaparsecs (x 1 billion parsecs) as a means of measurement. For instance Proxima Centauri, our nearest star, is located 1.296 parsecs away, while the distance from the Earth to the centre of the Milky Way is around 8.5 kiloparsec. The distance to the Virgo Cluster, which is a group of thousands of galaxies, on the other hand, is around 16 megaparsecs, meaning it would take light some 54 million years to reach it.

Star Magnitudes

Star magnitudes refer to a relative measure of brightness of astronomical objects as seen by an observer on Earth, with the brighter an object, the lower its magnitude value, and the brightest objects assigned negative values. Bearing in mind that on a clear night the naked eye’s limiting visibility will be around 6th magnitude, there are around 4,548 stars that can be seen with the naked-eye from each of the Earth’s hemispheres. This includes 2 stars with negative magnitudes; 6 stars of magnitude 0; 14 of magnitude +1; 71 of magnitude +2; 190 of magnitude +3; 610 of magnitude +4; 1,929 of magnitude +5; and 5,946 stars of magnitude +6.

Meanwhile, the Sun has an apparent magnitude of -27, the full Moon -13, the planet Venus -5, and Sirius in Canis Major, the night sky’s brightest star, of magnitude -1.46. Jupiter’s four Galilean moons, on the other hand, have a magnitude of between +4.6 and +5.6. In addition, 50mm binoculars will allow a viewer to see up to 9th magnitude, a 6″ telescope up to 13th magnitude star, while the Hubble Space Telescope can see distant objects with a magnitude of just +31.

Star Types

Hertzsprung -Russell (H-R) DiagramThere are seven main categories of stars, with their spectral type, temperature, color, luminosity, and evolutionary stage plotted on a useful graph called the Hertzsprung-Russell (H-R) Diagram. These types of stars can then be divided further into three main categories, namely young main sequence stars, old giant and supergiant stars, and smaller, faint, dying stars known as dwarfs. Here is a brief summary of the star types in order of temperature, together with their color, mass (M☉) and average luminosity (L) as compared to the Sun (sol):

O: Blue, 33,000K or more, = 16 M☉, = 30,000 L,
B: Blue White, 10,000–33,000 K, 2.1–16 M☉, 25–30,000 L☉,
A: White, 7,500–10,000 K, 1.4–2.1 M☉, 5–25 L,
F: Yellow White, 6,000–7,500 K, 1.04–1.4 M☉, 1.5–5 L,
G: Yellow, 5,200–6,000 K, 0.8–1.04 M☉, 0.6–1.5 L,
K: Orange, 3,700–5,200 K, 0.45–0.8 M☉, 0.08–0.6 L,
M: Red, 2,000–3,700 K, = 0.45 M☉, = 0.08 L,

An extra designations is then added after a star’s letter to signify how hot it is (0 the hottest, 9 the coolest), and a Roman numeral for its luminosity, according to the MKK system, as follows:

0: Hypergiants
Ia: Very luminous supergiants
Ib: Less luminous supergiants
II: Luminous giants
III: Giants
IV: Subgiants
V: Main sequence stars (dwarf stars)
sd: Subdwarf
D (prefix): White Dwarf

Rigel in Orion, for instance, is a B8Ia blue supergiant; Procyon in Canis Minor is a F5 IV–V white main-sequence star; the Sun is a G2V yellow dwarf star; Arcturus in Boötes is a K0 III red giant star; Antares in Scorpius is a M1Ib red supergiant; Proxima Centauri in Centaurus is a M6Ve Red Dwarf star; and Sirius B in Ursa Major is a DA2-5 white dwarf star.

Arcseconds and Arcminutes

Imagine measuring the width of an object by drawing an imaginary line from each of its two ends to your location, and then measuring the angle between these two lines. In essence, this is what astronomers do when they measure the apparent size of astronomical objects in relation to our position back on Earth, but instead of using degrees for measurement, they use angular sizes known as arcminutes and arcseconds. In this system, a circle is divided into 360 individual degrees, with each one of those individual degrees equivalent to 60 arcminutes, and each of those arcminutes equivalent to 60 arcseconds.

It is important to note, though, that astronomers are only ever measuring an object’s apparent rather than actual size in this way. The Sun is around 400 times bigger than the Moon, for instance, but their angular diameters are practically the same at around 1/2 degree, or 30 arc minutes (1,800 arc-seconds). Here are some other angular sizes to illustrate these measurements :

Jupiter: 50 arcseconds or 0.8 arcminutes (0.01 degrees)
Betelgeuse: 0.055 arcseconds or 0.0009 arcminutes (0.00002 degrees)
Proxima Centauri: 0.001 arcseconds or 0.00002 arcminutes (0.0000003 degrees).

Celestial Coordinates

Celestial SphereThose new to astronomy may be daunted at first by seeing celestial coordinate which use a grid system to describe the location of an object in space, such as this one which applies to the brightest star in the night sky, Sirius:

RA 6h 45m 9s | Dec -16° 42′ 58″

However, the system is based upon the concepts of longitude (position East-West of lines running from North Pole to South Pole, with reference point being the Prime Meridian through Greenwich at 0° longitude), and latitude (position North-South of Earth’s equator at 0° latitude). These coordinates are then projected onto a celestial sphere as follows:

– Right Ascension (RA) the equivalent of longitude, denotes an object’s location east of the Sun’s position during the March equinox, or the point where the Sun appears to traverse the celestial equator in a South to North direction during the year. One hour of right ascension is equal to 15 ordinary degrees of the sky, such that a star at RA 6h | Dec 0° would be separated by 15° from a star situated at RA 7h | Dec 0°

– Declination (Dec) the equivalent of latitude, measures an object’s location north (+90°) or south (-90°) of the celestial equator (0°), with the other coordinates denoted in arcminutes minutes ( ‘ ), and seconds ( ” ). In this way, declination allows astronomers to determine how far an object will rise in the night sky.

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