Light Time
The dome is open.
Light is fast. Very fast. The speed of light is roughly 186,000 miles per second or 3x1010 cm/s. For context, this means light can go around the world about 7 times in one second.
When it looks like the night is going to be clear, and I’m at home getting ready to leave, I always wonder where the light I’m going to catch in my telescope is right now.
Here is the answer. The average light time from Jupiter to Earth is about 35 minutes and from Saturn to Earth is about 1.2 hours (70 minutes). It takes me about 70 minutes from the time I get in my car to leave for my observatory until I take the first image. So when I get in my car the photons are roughly at Saturn’s orbit.
Mars is about 5 light minutes from Earth. So if I take a 5 minute exposure I basically collect a column of photons stretching from my telescope to Mars’s orbit.
Pluto is roughly 5.3 light hours from Earth. It’s not uncommon for me to do observing runs on a star 5 or 6 hours in length. During that observing session I basically collect a column of photons stretching to Pluto’s orbit.
If we call the solar system that within Pluto’s orbit, recent proceedings notwithstanding, light hurling through our little corner of the galaxy is in and out in less than half a day. By this definition our solar system is less than one 800th of a light year in diameter. By comparison, the disk of our Milky Way galaxy is thought to be about 100,000 light years in diameter, a hundred million times bigger than our solar system.
So the companions to the photons that land on my telescope, in their 100,000 year journey across the galaxy, are in our neighborhood just a few hours.
Monday, December 18, 2006