# How old is the Sun in Galactic years?

The Moon goes around the Earth, the Earth goes around the Sun, and the Sun goes around the center of the Milky Way. We know the Moon takes about 4 weeks to make its trip around the Earth, and that causes the Moon phases:

We also know that the Earth takes one year to go around the Sun, and that causes the seasons:

We also know that the Earth has been around for about 4.5 billion years, which means it has gone around the Sun about 4.5 billion times. Well, now I ask the question(s):

How long does it take the Sun to go around the Milky Way? How many times has it done that so far, and how many times will this happen before the Sun finally dies?

Well, we know that we travel in (roughly) a circle around the center of the Milky Way, and that our radius from the center is about 8 kiloparsecs, or roughly 26,000 light years. That means our Solar System (including the Sun) needs to travel a distance of 1.55 x 1018 kilometers to go around the Milky Way once. If we know how fast the Sun is moving, we can figure out how long a Galactic Year is. Well, we can both measure and calculate its velocity to be 220 kilometers/second, and so we can just do the math, knowing that there are 31,556,952 seconds in a Gregorian Year, and we find that it takes about 223 million years to make one galactic year.

So, if the Sun is 4.5 billion years old, that makes it about 20 galactic years old. If the Sun has a total lifetime of around 10 billion years, then it has a total galactic age of around 42 galactic years.

What? Did I just say the answer is 42?! Well, this means that one possible question is “What is the Sun’s lifetime in Galactic Years?”

1. #1 ethan
March 2, 2008

Wow! My article has just been roughly translated into Dutch! Look at these new words! Milky Way = Melkweg. Galactic years = galactische jaren. miljard jaar oud = million years old! lichtjaar = light year! This is awesome, Zal de Zon; thanks for bringing this to a new part of the world!

2. #2 shirley
March 13, 2008

good stuff here…please put me on mailing list.

3. #3 ethan
March 13, 2008

Shirley,

If you want on whatever list I have, just register by going to the link under the heading “Meta” on the right!

Ethan

4. #4 CODY
March 27, 2008

its a a nice site

5. #5 Joe blow
October 2, 2008

Get more F\$\$\$ Pictures

6. #6 Mark
United States
January 3, 2014

So, if the sun (4.5 billion years old) has gone around the galactic center 20 times in its life and we know that the galaxy is around 13.2 billion years old, that means that the galaxy hasn’t really “spun” that many times. The earlier back one looks, it must have been more dense at it’s center before the galactic arms took their present shape. And surely it would have spun faster the further back one looks. But in the big picture, and in a different time frame, it hasn’t spun all that much. Kind of like a spark being flung from a fire.

7. #7 isacsanthakumar
Kerala,India
October 20, 2014

14 billion years back Big Bang occured.How was it arrived at?The Galactic year is applicable to the Milky way.Can it be made applicable to the entire Universe?

8. #8 isacsanthakumar
Kerala,India
October 20, 2014

The Galactic year is applicable to the Milky way only.It is not applicable to the entire Universe,with innumerable Galaxies.If so,how can we say that the Big Bang occurred 14 billion years back.How was it arrived at?

9. #9 Wow
October 20, 2014

By projecting the trajectory and speeds of the contents of the universe to see what they do.

And they end up at the same spot around 14 billion years ago.

Not one jot of “galactic years” is required there, any more than Brhama Years are required. But you can state how long ago the universe began in Brahma years (nearly 2).

10. #10 jim henderson
Minneapolis,MN
April 23, 2015

Are there “seasons” in the cosmic year? Can we see a difference when we are on one “side” of the galaxy from the other side? Does the ecliptic ever change? Does the plane of our solar system (and others recently discovered) lie in the same plane as the galactic arms of the Milky Way?

11. #11 Vuma Theodore Mkhize
Jhb South Africa
October 16, 2015

Interesting …but I’m left with a question now. If the moon orbits the earth & the earth orbits around the sun & so does the sun around the galactic center then does the galaxy orbit around the big bang (the birth of space-time) or the center of the universe?
Did the birth place of space-time develop gravity later at some point after it occured.

12. #12 Warren
Perth WA - the ho.me of the computer at the centre of the square kilometre array
April 20, 2016

OMG Douglas Adams was right! The answer to the question of Life, the universe and everything IS 42!

13. #13 Terry Creley
Highland Village
July 1, 2016

This assumes the solar system spun into existence at its current locale. If instead we came out of a stellar nursery, then not only are we rotating around the galactic center, we’re also centrificalizing away from galactic central point. I propose that the sun has got to be a lot older than 4.5 boy to have been flung out 3/5ths the way to the edge of the galaxy.
I believe the Big Bang Theory is for those who really can’t handle the concept of infinity in both directions of all four dimensions.

14. #14 Albert
Namibia
November 27, 2016

Terry’s comment is too important to pass by. When looking at a galaxy it reminds me of a vortex where everything is gradually sucked into the center (reason for the many black holes at the center?). Thus our sun might have been formed on the fringes of the galaxy and through millions of orbits migrating towards the center. Whichever way, from inside to the outside or from the outside to the inside, 4.5boy is surely way too short. While the moon is orbiting the earth, the earth the sun, the sun the center of the galaxy, what prevents us to postulate that our galaxy might be revolving another center in a sort of super-galaxy. This would add another dimension to time as we understand it.

15. #15 Wow
November 28, 2016

“When looking at a galaxy it reminds me of a vortex where everything is gradually sucked into the center (reason for the many black holes at the center?)”

And when looking at a cloud, you can see a sheep. Doesn’t mean every cloud has a wool lining. You need more than a static appearance to make a claim of a dynamic in the system.

And, no, the black hole doesn’t suck anything in, any more than the earth sucks comets in, even though we get hit by cometary dust a lot.

16. #16 Carl Johan Calleman
February 24, 2017

Is there anybody who can answer when the Galaxy started to rotate with the rate of 250 million years? It can not have just started at this speed but gone from a stationary state and then approached the steady speed of 250 million years per round.

17. #17 Liam Arthur
United States
March 15, 2017

Is it OK if I use this quote for school, “So, if the Sun is 4.5 billion years old, that makes it about 20 galactic years old. If the Sun has a total lifetime of around 10 billion years, then it has a total galactic age of around 42 galactic years.” you will get credit and my classmates will check you out.