![]() Sirius is 23 times as luminous as the Sun, and about twice the mass and diameter. Sirius, the red giant star Betelgeuse, and Procyon in Canis Minor form a popular asterism known as the Winter Triangle. Your fist at arm's length covers about 10 degrees of sky, so it's about two fist-widths down. Follow the three belt stars 20 degrees southeast to the brightest star in the sky. To find the Dog Star, use the constellation Orion as a guide. The best time to see Sirius is probably in winter (for northern-hemisphere observers), because it rises fairly early in the evening. This wobble-called precession-is caused by the gravitational attraction of the Moon on Earth's equatorial bulge, and it gradually changes the locations of stars on the celestial sphere. This happens because the Earth has been wobbling slowly around its axis in a 25,800-year cycle. Sirius no longer marks the hottest part of summer, because it now rises later in the year. ![]()
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