They are North Pole and South Pole
two points of the earth's axis
the sun and the center of our galaxy, even the center of the known universe if it can be pinpointed
There are any number of such fixed reference points, but in real life the choices are made based on cultural or religious reasons rather than physical ones.Here are a few physical fixed reference points that one might use to base a calendar on.1. The winter solstice2. The summer solstice3. The Earth's perihelion (about January 4)4. The Earth's aphelion (about July 4).Of all the calendars in use on Earth, only one has is defined with any reference to an astronomical event, and it's pretty loosely connected; in the Hebrew calendar, Rosh Hashanah (the Jewish New Year) is at the New Moon closest to the fall equinox.
earth
The Equator
which are two basic point of reference on the surface of the earth
We have several reference points on the Earth's surface, for our convenience. The North and South Poles are a couple. The choice of Greenwich as the zero longitude, and of the International Date Line are another two. These points have no specific feature on the surface, they are decided on for the convenience of the users.
We have several reference points on the Earth's surface, for our convenience. The North and South Poles are a couple. The choice of Greenwich as the zero longitude, and of the International Date Line are another two. These points have no specific feature on the surface, they are decided on for the convenience of the users.
Latitude and Longitude
Motion movement and changes
The two points and the centre of the earth define a plane, and the intersection of this plane with the surface of the earth is a circle - the "Great Circle". The shortest distance between the two points is the smaller of the two arcs on this circle.
A line.
By definition, the word "collinear" means that the points lie in a LINE. The precise manner of line you are talking about depends upon the geometric frame of reference you are using. For example, in Euclidean geometry a line defined by two points is usually taken to be "straight" like the intersection of two planes; but in Global geometry such as the reality of the Earth's surface, a line defined by two points is curved. If you fly from New York to Tokyo, your line of flight is nearly a semicircle; we do not have the technology to travel "straight" from New York to Tokyo through the Earth's core.
Constellations are visible from various points on Earth throughout the year depending on where they are with reference to the night side of the Earth. The night side is the side which faces away from the sun, so as the Earth orbits, the direction of the sun from the Earth with reference to other stars, and consequently the night side of the Earth with reference to other stars, cycles yearly.
The shape of the Earth's orbit as it revolves around the Sun is an ellipse. This is a shape consisting of all points on a plane whose distances from two reference points (called foci, singular focus) add up to a given number. In every orbital ellipse, the body orbited (in the Earth's case, the Sun) is located at one of the foci.
Because the only points of reference we have when we look out is the earth and things on it within our view (such as mountains and distant trees etc.), and as we are also on the earth, these things don't move in relation to us as we look at the sun, so it 'must be' the Sun which moves. The same effect can be observed when you are on one of two trains which are stopped side by side - as one moves off, it can be difficult to tell which train is moving, because your points of reference have changed suddenly.
If the reference points are not correct, the location of any coordinate will be incorrect.