# Overview

## Celestial Sphere

When you look up at the night sky, you see hundreds of stars of varying brightness and sizes. The stars seem to be stuck to the underside of a dark dome which is called the ‘Celestial Sphere’. All stars are at different distances from Earth, but due to the lack of distance information when we look at stars, we see them projected onto the two-dimensional celestial sphere.

![](https://firebasestorage.googleapis.com/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MDuoOQZEQNrE9MzBv4b%2Fuploads%2F6xwgBLwW00gByCsNd2U4%2Ffile.png?alt=media)

{% embed url="<https://en.wikipedia.org/wiki/Celestial_sphere>" %}

As the night passes, the dome seems to slowly rotate around us carrying the stars with it. This is due to the relative motion between the Earth and the sky. As Earth rotates on its axis, we sense it in the form of the celestial dome rotating around us in the opposite direction.

![](/files/-MJ5RmYqN18lSxq1Svsv)

This apparent rotation of the celestial sphere causes the rising of the stars in the Eastern direction and their setting below the horizon in the Western direction. This effect of rising and setting is most pronounced in the stars that lie along the ‘Celestial Equator’ (a projection of Earth’s equator on to the Celestial Sphere) and least pronounced in the stars that lie close to the ‘Celestial Poles’ (an extension of Earth’s poles on to the Celestial Sphere). Consequently, stars that may lie exactly on the celestial poles never seem to rise or set for an observer on earth.

{% tabs %}
{% tab title="Northern Hemisphere" %}
![](/files/-MJ5q9sQR9hrDLZ3KYd4)
{% endtab %}

{% tab title="Southern Hemisphere" %}
![](/files/-MJ5qF5w8T-29m_D8-xg)
{% endtab %}
{% endtabs %}

The process of ‘Polar Alignment’ of the telescope mount is done by making the axis of rotation of the mount parallel to Earth’s axis of rotation and pointing it at the celestial pole (North celestial pole in the Northern hemisphere and South celestial pole in the Southern hemisphere). Once polar alignment is achieved, the mount will rotate along its axis (now parallel to Earth’s axis of rotation) in a direction opposite to Earth’s direction of rotation.

{% tabs %}
{% tab title="Northern Hemisphere" %}
![](/files/-MJ5vh7mZ2BKz2AA5-6o)
{% endtab %}

{% tab title="Southern Hemisphere" %}
![](/files/-MJ5wRXAJU5Cc2QXyxMV)
{% endtab %}
{% endtabs %}

{% hint style="info" %}
The images above are not drawn to scale and hence spacing between the two red dashed parallel lines is overly exaggerated. In reality, the stars are so far away that these two lines would overlap.
{% endhint %}

Polar alignment helps to accurately offset the motion of the stars on the celestial sphere caused by the Earth’s rotation. The mount’s rotation opposite to Earth’s rotation is observed as the mount ‘tracking’ the stars in the night sky.

![](https://firebasestorage.googleapis.com/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MDuoOQZEQNrE9MzBv4b%2Fuploads%2F3WG1kxQhPjsOvxLhH8uT%2Ffile.png?alt=media)

The equatorial mount can be rotated on two axis for polar alignment, the altitude and azimuth.


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