About 400 years ago, Galileo Galilei turned a 30 power Time Machine towards the heavens and discovered the moons of Jupiter. Can you just imagine what sort of scene that was back in Venice? When Galileo stared through his Time Machine, he was looking at light that left our Sun about 42 minutes earlier, bounced off the clouds and moons of Jupiter, and came back to Earth about a half hour later. Meanwhile, the light from the Pleiades star cluster, about 400 light years away, is just reaching our eyes now.
Light is very very very very very very very fast. For most people, 300,000 km/s is a good average speed (about 186,000 miles per second). Here are some approximations of how fast light can travel:
- 1 foot in one nanosecond (0.000000001 second)
- 1 km in 0.0000033 seconds
- 1 mile in 0.0000054 seconds
- Around the equator in 0.134 seconds
- From the moon to the Earth in 1.3 seconds
- Sun to Earth (1 AU) in 8.3 minutes (you should know this one!)
- Across the Milky Way in 100,000 years
- To the nearest galaxy that you’ve heard of (the Andromeda Galaxy) in 2.5 million years
- To the edge of the observed universe in 13,500,000,000 years
Yes, light travels very fast, but it’s still very slow. Imagine how long it would take you to fly around the world. In a supersonic jet going the speed of sound, it would take about 35 hours to make the flight. Light does this same trip in 0.134 seconds. That’s more than 940,000 times faster than your jet. Yet it will take light more than eight minutes to get to the Sun and more than four years to get to the next nearest star. That would be almost 15 years for your jet to get to the Sun, and about four million years to get to Proxima Centauri!!!
Space really is too big to think about how far away things are in distance, which is why we use measurements of time – the light year. A light year is the distance light can travel in one year, or about about 10,000,000,000,000 km. If I got my zeros and commas right, that should be ten trillion. So the Sun is about 0.000016 light years away (seems positively close, doesn’t it?), and Proxima Centauri is 4.3 light years away – 27,000 times farther away than our own Sun.
So when Astropotamuses talk about looking back in time with our Time Machines, we are being very sincere. The light from Proxima Centauri that left mid-January of 2007 is just now reaching my Time Machine. In mid-January of 2007, I was thinking about going to medical school. (My, times have changed – I am not a doctor.) Light that left the surface of the stars of the Pleiades star cluster 400 years ago, back when Galileo was looking up at Jupiter for the first time, is also just now reaching my Time Machine.
In a way, me looking up at the Pleiades connects me with the beginnings of modern astronomy, in that I know that what I’m seeing now started its 3,800,000,000,000,000 kilometer journey at the same time that Galileo said, “Mi sembra di vedere le lune di Giove!” (I just made that up, I dont think he really said “I think I see the moons of Jupiter,” but you never know.) So it’s sad to see them disappear this time of year, because where I live, they are best seen in the winter. On the other hand, it means summer is not too far away!