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How long is a galactic year?

In one galactic year, also known as a cosmic year, the sun orbits the Milky Way.
In one galactic year, also known as a cosmic year, the sun orbits the Milky Way.
(Image: © Shutterstock)

Humans are used to keeping time by measuring Earth's movement relative to the sun. But while Earth's trips around its star are noteworthy to life on our pale blue dot, that journey is pretty insignificant when compared with the epic voyage that carries the sun — and our entire solar system — around the center of the Milky Way

Orbiting the Milky Way galaxy just once takes the sun approximately 220 million to 230 million Earth years, according to Keith Hawkins, an assistant professor of astronomy at the University of Texas at Austin. 

In other words, if we were to measure time by this galactic "clock," Earth would be about 16 years old (in galactic, or cosmic years), the sun would have formed about 20 years ago, and the universe would be just about 60 years old. 

Related: Does the sun rotate?

The solar system's journey around the galaxy resembles Earth's orbit around the sun. But rather than orbiting a star, the sun circles the supermassive black hole that lies at the center of the Milky Way, Hawkins said. It exerts a tremendous amount of gravity on objects near the center of the galaxy, but it's the gravity exerted collectively by the material in the Milky Way itself that keeps the sun in its orbit.

"The sun is moving with enough speed — about 230 kilometers a second, about the equivalent of 500,000 miles per hour — that it continues to revolve around the center of the galaxy in sort of a circle" instead of getting pulled toward the black hole, he said. 

Our place in the galaxy

Compared with an Earth year, a galactic year represents time on a grand scale — but it's not a consistent measurement across the galaxy. What we Earthlings call a galactic year is specific to Earth's place in the Milky Way's spiral. 

"We would say that a galactic year is 220, 230 million years. Other stars in the galaxy, their galactic year is different," Hawkins said.

The galaxy is about 100,000 light-years across, and the Earth is about 28,000 light-years from its center. "If you imagine the galaxy as a city, the Earth is somewhere near the suburbs," Hawkins explained. For stars that orbit close to the black hole — the center of the "city" — a galactic year is relatively short. Out in the "suburbs," where our solar system lies, "the galactic years are a little longer," he said. 

Similar rules control variability in the length of a year between planets. For instance, Mercury, the innermost planet in our solar system, makes a complete orbit around the sun in about 88 Earth days. Uranus, the seventh planet from the sun, orbits the sun every 84 years, by Earth standards. And the distant dwarf planet Pluto takes 248 Earth years to finish one orbital cycle. 

While the physics of planetary orbits are similar to the mechanisms that shape the orbit of our solar system around the Milky Way, it's worth asking how astronomers have figured out the span of a galactic year. Hawkins says that it's actually pretty basic science that became clear in the early days of modern astronomy.

"It's mostly about watching stars move around the galaxy," he said. "You can watch stars move around the galaxy and deduce from the speed and direction of other stars." 

Editor's Note: This story was updated on Aug. 31 to note that the sun stays in its orbit around the Milky Way not just because of the black hole at the center of the Milky Way but also due to the gravity exerted collectively by the material in the Milky Way.

Originally published on Live Science.

  • kamikrazee
    I don't know how long a galactic year is in human terms, but this year seems like it is lasting forever.
    Reply
  • JamTop1105
    How do you include Uranus and Pluto, but forget Neptune?!
    Reply
  • OldSurferDude
    How could you write an article like this and not include a link to The Galaxy Song?
    Reply
  • G Shogun
    No it isn't.
    Reply
  • wifiPlotz
    Eventually what is left of the Sol system gets sucked into the super black hole at the center of the galaxy. Poof.
    Reply
  • Sanjay
    It is an informative article. But,in my opinion the appropriate title of the article should have been ' How long is a sun year ? 'because the article describes the revolution of the sun around Milkyway Galaxy in around 230 ‐240 million years and not the revolution of the Milkyway Galaxy around any cosmic object.
    Reply
  • wifiPlotz
    Sanjay said:
    It is an informative article. But,in my opinion the appropriate title of the article should have been ' How long is a sun year ? 'because the article describes the revolution of the sun around Milkyway Galaxy in around 230 ‐240 million years and not the revolution of the Milkyway Galaxy around any cosmic object.
    And now there was some article about the Andromeda galaxy's halo that is 'nudging' the Milky Way. I will have to read up on that. But yea, I did consider that galaxies could rotate, but our deepest picture of the universe shows them in linear direction, no rotation... :D
    Reply
  • Ashtin
    Derk1946 said:
    The solar system is actualy vibrating creating TIME and space time cycle ripples, one needs to have in depth knowledge of geometry to understand just how this all works. Each planet has its own pulsating ripple cycle "image" measure and the earth has the longest becouse its expanding and contracting both ways held in balance . The ancient gave names to all these images as being GODS. The first being Mercury or Hermes, (greek or roman) the fast one. other cultures having different names) but in overall time they are all in the same time frame, but in space ripple or viberation cycle time each is different. Using base 10 math is (9 planets and 1 sun) the overall time frame cycle is 86 (+-1) =87. A gravitational pulsating black hole exist in the centre expanding and contracting over time so the solar system is in affect always remains in balance but appears to move becouse of the ripples (expanding/contracting) this "differential" equation has the measure of (Pi). Eventualy all space time will fill up due this rippling motion and so will the empty VOID ..ie - Moment of completion as from dot to BLOT as the ancients referred to it. ONE can think of it as being like the nautilus shell found in the sea when the life within it has ended and gone leaving behind only the skeleton shell which It has built around its SELF as with all other creatures including human beings. The great pyramid of giza would be another example of such a thing no body remains have ever been found within it becouse its motivating creator no longer exists except in writtem records like the pharaoh but he never actualy built it himself skilled workers did. Life is an illusion functioning on differing levels in material, Spiritual and metophisical form, all this can be found in genesis in the bible. Science and religion functioning together using TIME and space time.
    True,but...
    Reply
  • Amoriko Savarias
    Dear Grant-Currin,

    First, I want to remind you about Vera Rubin's greatest work in studied Andromeda galaxy. She found that the movement of those stars is in the same angular speed. That's means, they move like a plate of pizza. All stars are in the same formation in a plate of the galaxy.
    Contradict to planetary movement in our solar system. Despite the farthest planet to Sun as the center moves slower, the farthest stars to the center of the galaxy will move faster. By that founding, she proposed the Dark Energy and the Dark matter. (CMIIW).

    In the galaxy-wide, we should use RAM (Revolve Around the Middle) as the galactic-wide time. In our Solar System, 1 RAM is about 220-230 million Earth years.

    If you want to meet someone from another star system, you must use the same time calculation that can be used galaxy-wide.
    Reply
  • Benjamin
    Amoriko Savarias said:
    Dear Grant-Currin,

    First, I want to remind you about Vera Rubin's greatest work in studied Andromeda galaxy. She found that the movement of those stars is in the same angular speed. That's means, they move like a plate of pizza. All stars are in the same formation in a plate of the galaxy.
    Contradict to planetary movement in our solar system. Despite the farthest planet to Sun as the center moves slower, the farthest stars to the center of the galaxy will move faster. By that founding, she proposed the Dark Energy and the Dark matter. (CMIIW).

    In the galaxy-wide, we should use RAM (Revolve Around the Middle) as the galactic-wide time. In our Solar System, 1 RAM is about 220-230 million Earth years.

    If you want to meet someone from another star system, you must use the same time calculation that can be used galaxy-wide.
    I'm not the brightest, but i just ripped a J & was thinkin', like carl sagan would... now if the water is going down the drain towards the milky way center so to speak, was a galactic year longer on its last year around the center? Will the galactic year progressively get shorter as we whip around faster towards the center?
    Reply