Astronomy, the Earth’s Rotation and the Seasons
All of the stars and planets in the night sky appear to move as the night progresses, but they don’t. In fact, this apparent movement is due to the rotation of the earth on its axis – the same rotation that causes the sun to move in the daytime sky. Over the course of one hour the earth rotates 15 degrees on its axis, and so the stars appear to move 15 degrees. For comparison, the full moon occupies about ½ a degree.
So the stars and planets rise in the East, and set in the West. They all rotate in an anti-clockwise direction about a point known as the celestial pole - an imaginary point directly above the earth’s north pole. For those of us in the Northern hemisphere, there is a star which happens to be almost exactly at this point. Polaris or the Pole Star is in Ursa Minor - the Little Bear - and all other objects appear to rotate around this one.
The Stellarium simulation below illustrates this nicely – notice how Polaris is the only object which remains stationary in the speeded-up night sky:
Objects near to the Pole Star are always visible from the Northern hemisphere – they never rise or set and can be seen all year round. These objects are known as circumpolar. In the example video above the bright star Vega in the constellation of Lyra is just circumpolar, and never quite sets.
Those objects which are not circumpolar are seasonal, and so are only visible at certain times of the year. This is because for much of the year, they occupy the same area of the sky as the Sun, and so are drowned out by the glare of daylight. Becoming familiar with the visibility of constellations and objects as the seasons progress is one of the many delights of astronomy.
The Phases of the Moon
The moon orbits around the earth, with an orbital period of around 28 days. The moon rotates about its axis just as the earth does, but due to the phenomenon of tidal locking the moon’s rotational period and its orbital period are the same, 28 days. This means that the same hemisphere of the moon is always facing the earth, giving the moon a constant “face” as viewed from the earth.
The effect is well explained in this video:
As the moon moves in its orbit its position relative to the sun changes, causing different parts of the moon’s earthward face to be lit. This gives rise to the familiar phases of the moon, as follows:
- New moon (unlit)
- Waxing crescent (evening object)
- First quarter (evening object – sets at midnight)
- Waxing gibbous
- Full moon (or second quarter – visible all night)
- Waning gibbous
- Third quarter (morning object – rises at midnight)
- Waning crescent (morning object)
- New moon again
It is important for an amateur stargazer to understand this rhythm, because – apart from being a fascinating subject in itself – it governs the best times for stargazing. In the two weeks between first and third quarter moon, the brightness of the moon is such that all but the brightest objects are rendered faint or even invisible, regardless of the quality of your telescope. Indeed, the brightness of the moon at these times can even be a problem for those observing the moon itself, necessitating the use of filters to reduce the glare.