Stars of Two Hemispheres

Fig 1 Looking south from Rousay 2005 February 7 at 05h 45m. A 15 sec. exposure 50mm f/1.4 ISO 350. Stationary camera. Antares appears as an orange star close to the horizon. John Vetterlein.

Dia 1 The constellation Scorpius at meridian passage. The position of the southern horizon (Lat. +59º) is shown bisecting the figure.

The twenty brightest stars in the entire sky are often loosely referred to as stars of the first magnitude.

This is misleading since the difference between the brightest (Sirius) and the faintest (Regulus) amounts to 2.8 on the magnitude scale.

In terms of relative brightness, Sirius is approximately twelve times as bright as Regulus. (See archives on star magnitudes.)

If we look for a more precise definition, then we should say that stars with magnitudes between 0.0 and 1.0 are first magnitude stars.

There are twelve such stars, Vega being the brightest, Spica the faintest. There are just four stars brighter than this of which only one, Arcturus, is a northern hemisphere star.

There is some variability in the light of some of the stars in our list but nothing so drastic as to challenge their right to be called the brightest stars.

The northern hemisphere is less well served with bright stars than the southern hemisphere. From the latitude of Orkney fifteen of the brightest stars are visible but only thirteen are seen with relative ease. Fomalhaut, with a declination at present of -29.6º is a severe challenge since it rises just 1.6º above the southern horizon when it may be glimpsed for a short time during autumn.

Antares, with a declination of -26.4º, is an interesting case. It culminates at midnight on June 1. For this reason in the northern hemisphere Antares is generally known as a summer star. But twilight throughout June is strong which means that Antares cannot be seen in a dark sky from these latitudes.

However, our northern winters allow us another opportunity to see Antares owing to the darkness of the early mornings.

Throughout January and most of February the bright red star may be seen low down in the relatively dark, southern sky shortly before dawn Fig. 1 shows the sky looking SSE on 2005 February 7 at 05h 40m from Rousay.

Many people regard Orion as the most spectacular constellation in the entire sky.

It straddles the two hemispheres more or less equally with the belt stars lying close to the celestial equator. Scorpius, with Antares as its principle star, is another fine constellation when seen from lower latitudes. (A good view may be had from Southern France.) It also a constellation, like Orion, which bears some resemblance to the figure it is named after. Dia 1 shows the principle stars of Scorpius with the position of our southern horizon drawn in. Antares is depicted as the heart of the creature: the head ranges above the horizon, the tail curling upward to the left, below.

There is nothing in our northern celestial hemisphere to rival the Southern Cross, containing as it does two bright, first magnitude stars. The Plough (part of Ursa Major), though less spectacular in terms of bright stars, is however readily identifiable with its pointers directing us towards Polaris. There is no equivalent southern pole star.

JV 15/05/05