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.

Light year
A light year is simply measured as the distance that light travels in one year.Light year unit is used to represent distances in the Milky Way and distances to closer Galaxies.

Light travels the distance about 186,287.5 miles per each second, so we will get the distance travelled by light within one year in following way.

One light year = Total seconds in year * Speed of light per second
Total seconds in year = Seconds for each day * days in year.

Seconds for each day = Hours in day * Seconds in hour.
................................ = 24 * 60 * 60
................................ = 86,400

Days in each year = 365.25 ( averaged )

Total seconds in year = 365.25 * 86,400
................................ = 31, 557, 600 seconds in a year

So,using this we can calculate one light year as follows,
One light year = 31,775,600 * 186,287.5
..................... = 5,878,786,100,000 miles !

So its almost about 6 Trillion miles.
And its Equals to about 63,241.1 Astronomical Units.

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

• Earth is 8 light minutes away from Sun.
• Pluto is 5.7 light hours from Sun.
• Our Galaxy The Milky Way is about 100,000 light-years across.
• Closest Galaxy to us i.e. Andromida Galaxy is 2.5 million light-years away.
• Closest start to us, other than Sun is Proxima Centuri and that is 4.2 l years away.

But Universe is so big that for some far distances even higher units like light year becomes inconvinient.


Magnitude.
Magnitude is a unit used to measure the brightness of any celestial object. Its a logarithmic scale.
The apparent magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude. Sun has the highest magnitude (i.e. lower value) and it is –26.73, Hubble telescope can see objects up to magnitudes of 30, which are not only unvisible to naked eye but are really really faint.

Below is the Magnitude of some of the familiar objects in sky
Full Moon has Magnitude of -12.6
The Crab nebula has Magnitude of -6.0
Maximum brightness of Venus has Magnitude -4.6
Maximum brightness of Venus -2.9
Maximum brightness of Jupiter -2.9
Brightest star other than Sun has Magnitude of -1.47
Our closest Galaxy Andromeda Galaxy has Magnitude of 3.4