Some of Us Are Looking at the Stars

Although fixed in shape, fancifully, as ploughs, bears, archers and the like, the constellations visible to us change with the seasons. Obviously we cannot see any stars during daylight hours when we are facing the sun. The stars we can see now, in mid-winter, are only those visible from the dark side of the Earth when we are turned away from the Sun. As the Earth moves to its mid-summer position on the opposite side of the Sun a completely different set of stars become visible to us and the winter stars we see now will be obscured by sunlight during the day. That is why Orion is a winter constellation while Scorpius is seen in summer. As we slowly transition from winter to summer the stars visible at night migrate westwards in the sky. The winter constellations gradually sink below the western horizon to be replaced by summer ones rising in the east.

These perceived changes in the appearance of the heavens depend on two things the Earth is doing. It spins on its axis, giving us night and day and it orbits the Sun (at 30km a second) giving us the seasons. Because of this rapid movement of Earth around the Sun, the position of the Sun in the sky as seen from Earth changes slightly every day. For this reason sunrise actually occurs 4 minutes after the Earth has completed one single 360º rotation. For obvious practical reasons we set our clocks to sunrise and sunset – not star (sidereal) time. We divide the day into 24 hours but a complete rotation of the Earth only takes 23 hours and 56 minutes. This means that the stars appear to move westwards by 4 minutes every night. In a year this adds up to 24 hours and the stars return to the same position in the sky every 12 months as the Earth returns to its original position.

In cosmic terms the Sun is quite close to us. An astronomical unit (AU) is the distance between the Earth and the Sun. It amounts to 150 million km and it takes sunlight 8 minutes to reach us. Because the stars are so much further away from us than the Sun their positions within any given constellation appear to be fixed in the night sky despite our own journey around the Sun. By contrast the planets of our solar system wander about through the constellations in an apparently random way that intrigued ancient observers.

Obviously those fanciful pictures the stars make in the sky are not delineated by points of light stuck onto the inner surface of a dark hemisphere but to stars which vary enormously in distance from us. Although our earthly position in the solar system changes by only 2 AU every six months this does affect the relative position of stars due to the phenomenon of parallax. Parallax is the change in position of close objects relative to distant objects as the point of observation changes. Try holding one finger up against a distant background with one eye open. Now close that eye and open the other one. The change in the position of your finger is due to parallax.

In the night sky the differences are so tiny that the existence of parallax shift was debated by astronomers for many years. Indeed, the apparent lack of any observable parallax was used to argue against Copernicus’ heliocentric model of the solar system; the problem was that early astronomers like Tycho Brahe could not conceive of distances so great that the measured shift would be infinitesimal.

Using very precise measurements a closer star will be seen to move relative to more distant ones as the Earth moves by 2AU around the Sun. By measuring this minuscule shift in position and by knowing Earth’s distance from the Sun it is possible to use trigonometry to calculate the distance to the nearer star. The first scientist to do this was Friedrich Bessel in 1838. The unit of length derived from this calculation is the “parallax second” or parsec (pc) which is defined as the distance at which 1AU subtends an angle of one arcsecond. An arcsecond is a mere 1/3600th of a degree. Expressed in other more familiar terms a parsec (pc) is 3.26 light-years or 206,000 AU. The nearest star, Proxima Centauri, is 1.3 pc away.

Parsecs are used to quantify distances to closer objects within our galaxy while kiloparsecs (kpc) are used for more distant intra-galactic things. Beyond our Milky Way we use megaparsecs (Mpc) for distances to the closer galaxies and gigaparsecs (Gpc) for quasars and more distant galaxies. The colossal size of these units is not easy to comprehend.

There is a famous mistake in George Lucas’ Star Wars (1977) when Han Solo refers to the Millennium Falcon having done the ‘Kessel Run’ in less than 12 parsecs. As discussed above, the parsec is a measure of distance not time despite ‘sec’ appearing in the unit’s name – and for 40 years Lucas has had trouble explaining it away.