Astronomy Answers: The Starry Sky: 00:00 Hours Sidereal Time

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This page shows the starry sky for 50° north latitude at 0 hours sidereal time. It is approximately 0 hours sidereal time at 0 hours local time at the end of September, at 18 hours local time at the end of December, at 12 hours local time at the end of March, and at 6 hours local time at the end of June.

The black disk indicates the sky, in stereographical projection. The circle at the outer edge of the disk is the horizon. The middle of the disk is the zenith. North is up, east to the left, south at the bottom, and west to the right. The white dots are stars of magnitude 3.5 and brighter. The larger the dot, the brighter the star.

The white lines are lines of constant right ascension and declination in the equatorial coordinate system. The white numbers indicate that right ascension (upper) and declination (lower).

The stars appear to rotate around the northern celestial pole at a declination of +90°, at the point inside the smallest white circle where several lines intersect. At 0 hours sidereal time, the right ascension of 0 hours is by definition due south.

At a geographical latitude of b degrees north the northern celestial pole is always at b degrees above the horizon and only stars with a declination of b degrees can stand straight above your head (in the zenith). The largest altitude that a star with a declination of d degrees can reach is then d + 90° - b (if this is not greater than 90°). The smallest altitude is d + b - 90°. Stars with a declination of 90 - b degrees are circumpolar: they are always above the horizon. Stars with declinations between b - 90 and 90 - b degrees are sometimes above the horizon. Stars with declinations below b - 90 degrees are always below the horizon.

The same story holds also in the southern hemisphere, if you replace "north" with "south", put a minus sign in front of all positive declinations, and remove the minus sign from all negative declinations.

At 50° north latitude, the north celestial pole is then at 50° altitude. Stars with a declination of 50° kan be in the zenith, and a star with a declination of d degrees can be at altitudes between d + 40° and d - 40°. Stars with declinations above 40° are then circumpolar, and stars with declinations below −40° are never seen.

Because there is a fairly bright star close to the northern celestial pole (namely the Pole Star or Polaris) we can estimate our latitude (in the northern hemisphere) by measuring the height of the Pole Star above the horizon.

The red line indicates the ecliptic, the approximate path of the Sun, Moon, and planets. The purple line shows where the Milky Way is. The Milky Way passes almost through the zenith at 0 hours sidereal time.

Four constellations are indicated in blue. At 0 hours sidereal time, Orion is just rising in the east, the Great Bear (Ursa Major) is low in the north, Cassiopeia is almost overhead, and the Swan (Cygnus) is in the west.

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Last updated: 2021-07-19