Sirius - Visibility

Visibility

With an apparent magnitude of −1.46, Sirius is the brightest star in the night sky, almost twice the brightness of the second brightest star, Canopus. However, it is not as bright as the Moon, Venus, or Jupiter. At times, Mercury and Mars are also brighter than Sirius. Sirius can be seen from almost every inhabited region of the Earth's surface, with only those north of 73 degrees unable to see it. However, it does not rise very high when viewed from some northern cities, reaching only 13° above the horizon from Saint Petersburg. Sirius, along with Procyon and Betelgeuse, forms one of the three vertices of the Winter Triangle to observers in the Northern Hemisphere. Due to its declination of roughly −17°, Sirius is a circumpolar star from latitudes south of 73° S. From the Southern Hemisphere in early July, Sirius can be seen in both the evening where it sets after the Sun, and in the morning where it rises before the Sun. Due to precession (and slightly proper motion), Sirius will move further south. From AD 9000 Sirius won't be visible anymore from northern and central Europe and in AD 14000 (when Vega is close to the North Pole) its declination will be -67º and thus will be circumpolar throughout South Africa and in most parts of Australia.

Sirius can even be observed in daylight with the naked eye under the right conditions. Ideally, the sky should be very clear, with the observer at a high altitude, the star passing overhead, and the Sun low down on the horizon. These observing conditions are more easily met in the southern hemisphere, due to the southerly declination of Sirius.

The orbital motion of the Sirius binary system brings the two stars to a minimum angular separation of 3 arcseconds and a maximum of 11 arcseconds. At the closest approach, it is an observational challenge to distinguish the white dwarf from its more luminous companion, requiring a telescope with at least 300 mm (12 in) aperture and excellent seeing conditions. A periastron occurred in 1994 and the pair have since been moving apart, making them easier to separate with a telescope.

At a distance of 2.6 parsecs (8.6 ly), the Sirius system contains two of the eight nearest stars to the Solar System and is the fifth closest stellar system to ours. This proximity is the main reason for its brightness, as with other near stars such as Alpha Centauri and in stark contrast to distant, highly luminous supergiants such as Canopus, Rigel or Betelgeuse. However, it is still around 25 times more luminous than the Sun. The closest large neighbouring star to Sirius is Procyon, 1.61 parsecs (5.24 ly) away. The Voyager 2 spacecraft, launched in 1977 to study the four Jovian planets in the Solar System, is expected to pass within 4.3 light-years (1.3 pc) of Sirius in approximately 296,000 years.