First look
The idea, in plain words
The stars of a winter evening are not the stars of a summer one. In December Orion stands high in the south after dark. By June he has gone, and you could search all night without finding him. He has not moved. We have.
The Earth goes round the Sun once a year. At night you are on the side of the Earth that faces away from the Sun, looking out into space. As the Earth moves round its orbit, that outward view swings slowly round too. In December it points towards Orion. Six months later it points the opposite way, and Orion is up in the daytime, lost in the glare of the Sun.
Seen from the Earth, the Sun seems to creep along in front of the stars, about one degree a day, and returns to where it began after a year. Its path is called the ecliptic. The constellations it passes in front of are the zodiac, and it spends about a month in each. You never see the one the Sun is in, because it is up only by day.
The Moon and the planets keep close to the same path. That is because the Solar System is nearly flat, so from inside it everything appears along one line across the sky. From a very dark place you can even see that line as a faint cone of light after dusk. It is sunlight scattered by dust lying between the planets.
The sky also changes far more slowly than that. The Earth’s axis wobbles like a spinning top, once round in about 26,000 years. So the pole of the sky drifts among the stars, and so does the Sun’s place on any given date. The star signs in a horoscope are where the Sun was in ancient times. Today it is about one constellation further on.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Ecliptic
- The line the Sun appears to follow against the stars through the year, which is the plane of the Earth's orbit seen from inside it.
- Zodiac
- The band of constellations that the ecliptic runs through, where the Sun, Moon and planets are always found.
- Constellation
- One of the 88 named areas into which the whole sky is officially divided.
- Precession
- The slow circling of a spinning body's axis, like the wobble of a spinning top.
- Celestial pole
- One of the two points in the sky straight above the Earth's poles, around which the stars appear to turn.
- Equinox
- One of the two days each year when the Sun stands over the equator and day and night are close to equal everywhere.
- Sidereal day
- The time a planet takes to turn once measured against the stars.
Where this comes from
- NASA, From Stargazers to Starships: the path of the Sun, the ecliptic
- NASA, From Stargazers to Starships: precession
- NASA Space Place, Constellations and the night sky through the year
- ESO, Zodiacal light over La Silla
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.