The Solar System’s Rotation About the Center of the Milky Way

Question:  I have a question about galaxies, how do scientist know about the galactic year and how long it takes? Well people think that are universe is only 14 billion years old has our solar system gone through a galactic year?  — Lily

Answer:  Well, we can measure the rotation speed of the Milky Way Galaxy, which we find to be about 270 kilometers per second (or about 168 miles per second).  It takes about 200 million years for the Milky Way to complete one rotation.  I believe that this is what you are referring to as the “galactic year” (I have also seen it referred to as the “Cosmic Year”).  Since the universe is about 14 billion years old, our solar system has gone around the center of the Milky Way galaxy a few times.  Note, though, that the solar system is quite a bit younger than the universe: its age is about 4.6 billion years.  So, the solar system has gone around the center of the Milky Way galaxy about 4.6 billion / 200 million = 23 times.

Jeff Mangum

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3 Responses to The Solar System’s Rotation About the Center of the Milky Way

  1. Jan Slowak says:

    Hello, I am a mathematician, and would like to do a statistical analysis of the data that determines galactic rotation. Is there a database of measured data of stellar distances, redshift etc? For example, for our own Galaxy. Greetings Jan Slowak

  2. Steve Markiewicz says:

    Hello,
    As a layman , I am wondering how the speed of the solar system was worked out and what was the equation used?

  3. Jeff Mangum says:

    The velocity of the Earth in our Galaxy is a rather complicated measurement to make, and really is done done with a single equation. What we do is to take information about the relative motions of all of the stars that we can see in our Galaxy, a measurement of the distance from Earth to the center of our Galaxy (which is the point about which the Sun orbits in the Galaxy), and construct a model of these distances and motions. The star motions and distances we measure tell us that we are living in a disk-like galaxy, and that we are at a distance of about 8 kilo parsecs (or about 2.5 x 10^(17) km) from the center of that disk-like galaxy.

    Jeff Mangum

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