In an expanding universe, is Earth getting farther from the sun?

No. While astronomers do believe that the universe has been expanding since the Big Bang, this expansion works on the largest of scales, the scale of the galaxies. In other words, our solar system – our sun and its family of nine planets – is not expanding.

Earth is located 150 million kilometers – about 93 million miles – or 8 light-minutes from the sun. It’s thought to have been located at this distance from the sun since our solar system was born, some four-and-a-half billion years ago. So the sun isn’t getting farther from Earth. And, likewise, our sun isn’t getting farther from other stars in our own galaxy.

Why don’t the solar system and galaxy expand, while the universe as a whole does? The solar system and galaxy are held together gravitationally. Our Milky Way galaxy is a collection of hundreds of billions of stars. It’s thought to be one of billions of galaxies in the universe.

Now we’re at the scale that astronomers talk about when they speak of the “expanding universe.” Our galaxy is getting farther from other galaxies – every galaxy is. There are billions of galaxies, and they are all moving away from each other. In that sense, the universe is thought to be expanding.

October 29, 2009

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