Common units used in astronomy.

While watching astronomical programs on TV or while reading news or article about any astronomical events, we always come across some of the different units of measurement. We don't use these measurement units in our everyday life since they represent very small or very big numbers which we hardly deal with in our normal day to day life. These units are quite simple to understand as they based on very simple concepts in geometry or physics. These units represents distances, time, brightness, temperature and many other entities in the gamut of astronomy.
So Let's see some of them one by one.

AU or Astronomical Unit
It is the Unit for measurement of astronomical distances.
One Astronomical Unit is approximately the mean distance between the Earth and the Sun.















As you can see in the figure, one Astronomical Unit is a measure of a distance between Sun to the orbit of the earth. Since earth's orbit is not exactly circular, it keeps varying as earth moves through its orbit and thus its distance from the Sun, So the one AU value is a averaged from max and min value of the distance from earth to Sun.
Its value is 149,597,870 km (about 93 million miles).
The AU is most commonly used for the distances of objects with in our solar system. Pluto, the last planet in the solar system is found at an average distance of 39.47 au from the Sun. So its roughly more than 39 times away than that of a distance between earth and sun.

These are the distances in terms of AU for some of the known objects in sky.

• The Earth is 1.00 AU from the Sun
• Mars is 1.52 AU from the Sun
• Jupiter is 5.20 AU from the Sun
• Proxima Centauri (the nearest star to Earth, excluding our own Sun) is approximately 268 000 AU away from the Sun
• Sun's gravitational field's influence (i.e. Hill/Roche sphere) is 125,000 AU away. This distance is roughly 1.8-2.0 light-years
• And our own Sun is 1.7 × 10⁹ AU away from the center of the Milky Way.