An aspect of astronomy that has always fascinated me is that when we look at the sky, every object that we see above the Earth’s atmosphere, we see as it was in the past. Let me explain.
The sun we see in the sky is not the sun as it is now, but rather the sun as it was about eight minutes ago. (Never look directly at the sun, as it will damage your eyes.)
The sun is about 93 million miles from Earth, and it takes time for light to travel that distance. The speed of light in a vacuum is about 186,282 miles per second, so to travel the 93 million miles to Earth takes about 500 seconds, or 8.32 minutes. Therefore, we see the sun as it was 8.32 minutes in the past.
Right now Mars is about 2.55 times farther from Earth than the sun, so it takes about 21.22 minutes for light to travel that distance. If we were to express the distance between Earth and Mars in miles, it would be a very large number. We can use smaller numbers if we express the distance in light minutes, the distance light travels in one minute; instead of saying 237 million miles, we can say 21.22 light minutes.
As we move farther from Earth, the numbers get bigger. The current distance to Jupiter is 35.92 light minutes. Right now we can find Jupiter in the southeast sky when it gets dark in the evening. Since it is 35.92 light minutes away, we see as it was 35.92 minutes earlier.
The same is true for the dwarf planet Pluto. It is currently 32.85 times farther from Earth than the sun, so we change from light minutes to light hours. Pluto is now 4.55 light hours from Earth, meaning that means we see Pluto, not as it is now, but as it was 4.55 hours ago.
How far is it to the closest star? Other than the sun, the closest star to Earth is Proxima Centauri. This star is in the Alpha Centauri system, which is only visible from the Southern Hemisphere.
Even though this is the closest star to Earth other than our sun, the distance is too great to express in light hours. It is also too far away for us to use light days or even light months, so we use light years: the distance light travels in one Earth year, which is 5,878,499,814,210.01 (five trillion, 878 billion, 499 million, 814 thousand, 210.01) miles.
As you have probably surmised, if we were in South America looking at Proxima Centauri, we would see it as it was 4.22 years ago, the number of light years between Proxima Centauri and Earth. In other words, the light we could see today would have left Proxima Centauri and traveled through space nonstop for 4.22 years.
Our solar system is part of the Milky Way galaxy, a vast collection of stars, dust and gases. This disk of stars is about 100,000 light years across. That means that if we started at one edge and traveled at 186,282 miles per second with no stops, it would take us 100,000 years to get to the other edge. If we could stand above one edge and see the light from the far edge, we would be seeing that edge as it was 100,000 years before.
Our solar system is not in the center of the Milky Way. It is about 26,000 light years out from the center in what is known as the Orion Arm.
Another unit of distance that astronomers use is the parsec. A parsec is 3.26 light-years. The distance across the Milky Way galaxy can be stated as 100,000 light years or about 30,000 parsecs. Our solar system is about 8,000 parsecs from the center of the galaxy.
To make the numbers take a smaller form, we can use the units of kiloparsecs and megaparsecs. A kiloparsec is 1,000 parsecs, and a megaparsec is 1 million parsecs. Expressed in this way, the Milky Way galaxy is about 30 kpc across, and we are about 8 kpc from the center of the galaxy.
Moving still farther away, the Andromeda galaxy is our closest sister galaxy. It is 789 kpc, or 2.5 million light years, from Earth. At a dark site on Earth, we can often see the Andromeda galaxy with the naked eye. We are looking at light that has been traveling nonstop for 2.5 million years — the galaxy, not as it is now, but as it was 2.5 million years ago.
Remember, Andromeda is one of the closest galaxies to Earth. We can also see light from objects that are billions of light years away.
The universe is a really, really, really big place!