Can you answer the following questions? 

1. What is a year?

2. How many days are there in a year?

 
In the last class, we discussed the rotation of the Earth. While the Earth rotates around its axis, it also moves around the Sun. The motion of the Earth around the Sun is called ‘revolution’. Revolution means to move around something in an orbit

The Orbit of the Earth
While revolving around the Sun, the Earth follows a certain path; this path is called the Earth’s orbit. The Earth rotates a little more than 365 times around its own axis when she completes one revolution around the Sun. That is, each revolution takes about 365 days and 5.56 hours. Can you guess what we call this period? We call this period a year. The length of a year on the Earth is 365 days¹.

What is shape of the Earth’s orbit?
The shape of the Earth’s orbit is slightly elliptical (an ellipse is like an elongated circle). But this ellipse is not much elongated; so the difference is not easily detectable.

How elliptical is the earth's orbit?
The shortest distance (perihelion) between the Sun and the Earth is 147,098,290 km, and the longest distance (aphelion) between the Sun and the Earth is 152,098,232 km. The ratio between these two is less than 1.034. So the Earth's orbit is almost circular.

You might have seen an orbit in the shape of an elongated ellipse. A circle looks like an ellipse when watched from an angle. Take a bangle or any thin ring and view it (or try to draw it) from different angles. When viewed from top it looks like a perfect circle, when viewed from an angle it looks elliptical and if you look at it exactly from the side, it will look like a line. Many a times the diagram of the Earth’s orbit is drawn as if the viewer is watching it from an angle other than 90^° or 0^° (neither from above nor from within the plane of the orbit). If seen from within the plane of the orbit, it would look like a line (Figure 1).
 

Figure 1: The Sun-Earth system from three different perspectives

(a: as seen from within ecliptic, b: as seen from above North Pole, c: oblique view)

¹We combine the remaining 6 hours for four years and add an extra day in February once in four years. This is known as leap year.