by Amy Aponte
When you gaze into the night sky, you are looking into the past. To understand how this is possible, we have to think about the immense vastness of space. Exactly how big is space and just how far are space objects from us here on Earth?
First, let’s talk about our (Earth’s) position in space. Earth is one of eight planets that orbits the Sun in the Solar System, and the Solar System is located on one of the arms of the spiral Milky Way Galaxy (Figure 1). Earth is about 93 million miles away from the Sun and 2.7 billion miles from Neptune, the outermost planet in the Solar System. Now, I know that millions and billions of miles seems almost inconceivable to us on Earth, but that is only the beginning. Space objects within our Solar System are, in fact, relatively close to Earth. Space is so vast that for space objects outside of our solar system, scientists must use light years to describe distance.
Technically speaking, one light year is equal to the distance that light travels in one year, or almost 6 trillion miles! Scientists believe that the observable universe, or the region of space visible to us from Earth, stretches about 46 billion light years in every direction from us. Beyond that, it is difficult to say with certainty how big the universe stretches, but many scientists agree that it is at least 250 times larger than the observable universe. For comparison, the Milky Way galaxy is only about 100,000 light years across.
So, what do these extremely large distances mean for how we see space objects from Earth? Let’s take the Sun and compare it to the Milky Way’s nearest spiral galaxy, Andromeda (Figure 2). This galaxy is about 2.5 million light years away from Earth, meaning it takes 2.5 million years for the light from Andromeda to reach us here on Earth. That is astronomically immense compared to the 8.3 minutes it takes light from the Sun to reach us! Incredibly, Andromeda is the only object outside of the Milky Way that is visible to the naked eye (Learn more about how you can find Andromeda in the night sky here). Though not quite as far, Proxima Centauri, the nearest star to our sun, lies 4.3 light years from Earth (Figure 3), while the brightest star in the night sky, Sirius, is 8.6 light years away.
If you look up at the night sky tonight, you will likely catch a glimpse of the moon, stars, and maybe even other planets. Whatever you may see, remember that you will be taking a peek into the past, whether that is 1.3 seconds (the moon) or 2.5 million years (Andromeda Galaxy).
