Why the night sky changes through the year

Each star rises about 4 minutes earlier every night, which adds up to 2 hours a month.

Same time, different stars

Go out at 9 p.m. in January and Orion stands high in the south. Go out at 9 p.m. in July and Orion is nowhere to be seen; Scorpius and the Summer Triangle have taken its place. The stars have not moved. What changes is where Earth is on its orbit, and so which direction the night side of Earth faces. At night we look away from the Sun, and over a year that direction sweeps all the way round the sky.

Earth needs 365.25 days to go round the Sun, so it moves about 360° ÷ 365.25 ≈ 0.986°, roughly one degree, along its orbit each day. From Earth this looks as if the Sun slides one degree a day eastward against the background stars. To bring the Sun back to the meridian each day, Earth must turn about 361°, not 360°. That extra degree takes about 4 minutes, which is the difference between the 24-hour solar day and the 23-hour 56-minute sidereal day.

Our clocks follow the Sun, so the stars run fast against them. A star rises about 4 minutes earlier each night: half an hour earlier after a week, 2 hours after a month, 6 hours after three months. After a full year the gaps add up to 24 hours and the star is back where it started at the same clock time. Try it in the model: keep the time at 21:00 and move the date forward one month at a time. The sky jumps by about 30°, two hours of turning.

The ecliptic and the zodiac

The Sun's yearly path among the stars is the ecliptic. It is tilted 23.44° to the celestial equator because Earth's axis is tilted by the same amount, the figure in NASA's Earth fact sheet. Turn on Ecliptic in the model to see it as a gold line. The constellations the ecliptic crosses form the zodiac. The traditional list has twelve, but with the IAU boundaries the Sun also passes through a thirteenth, Ophiuchus, the serpent bearer. The IAU's education office lists all thirteen, and EarthSky puts the Sun in front of Ophiuchus from about 30 November to 18 December. The constellations are not equal in size either: the Sun spends about a month and a half in front of Virgo and only about a week in front of Scorpius. Astrological signs are something different, twelve equal 30° slices of the ecliptic, and have no scientific basis.

The tilt of the ecliptic also explains the seasons in the sky. At the June solstice the Sun sits 23.44° north of the celestial equator, so at Seoul its noon altitude is 90° − 37.57° + 23.44° ≈ 75.9°. At the December solstice it is 23.44° south and the noon Sun reaches only about 29.0°. A high Sun stays up longer and heats the ground more directly, which is summer. The same geometry runs the night: the winter Sun is low, so winter nights are long, and the ecliptic's winter-night stretch, through Taurus and Gemini, rides high.

A calendar of the evening sky from Korea

The table was checked against the model for Seoul, and you can check it yourself: set 15 April at 21:00 and Leo stands near the meridian, almost 70° up; set 18 October and Pegasus and Cygnus are high while Orion has not yet risen. Precession slowly shifts all these dates. Sky and Telescope notes that the dates on which the Sun enters each constellation slip by about one day every 70 years.