How do we Measure that Far?

February 26 is Astronomy Night at Tye Preston Memorial Library in Canyon Lake.  We start at 7:30 INSIDE with a night sky and safety orientation. Last month we enjoyed the company of 60 guests, so come on out and see what’s in our local universe. We will be there, rain or shine, outside at telescopes if it’s clear, inside with a presentation if the sky is not clear.

Humans like to categorize everything, including distances of stuff from one point to another. It’s not a big deal to measure the distance from the fridge to my favorite chair (19 feet). A tape measure works just fine.  But what about measuring the distance between us and the Moon, Sun, other planets, and beyond?

Measuring distances to the Sun and Moon were two of the first big challenges for early astronomers. Enter geometry. In the second century BCE a Greek astronomer and mathematician named Hipparchus came up with an idea. When you look as something from different places, closer things seem to shift in position more than farther things. A quick example of this is by extending you arm out with your thumb up. Keeping your head stationary, look at it with one eye, then the other.  Your thumb seems to shift in position more compared with things farther away.  He thought, hmmm, knowing the distance between your two different places (your eyes in this example) and the angles (they are equal) from those places to the object in question gives the distance. How? You end up with a triangle. You have your distance between viewing points (baseline) and the angles. Hipparchus developed a formula to determine the length of the other two triangle sides and voilà, PARALLAX is born!

Astronomers have used parallax for centuries.  The Earth’s orbit around the Sun is used, and specialized satellites named Hipparcos and Gaia were used as the baseline. Parallax is a powerful way to measure distant objects, but it is limited by baseline length. The farther away an object is, the narrower the angles become, to a point where the potential error in measurement becomes too large. Gaia’s baseline for example was only two million miles greater than Earth’s orbit. It is very accurate up to about 1000 light years away from Earth. That’s a drop in the bucket! Maybe a satellite in the orbit of Neptune can give a long enough baseline?

There are non-direct ways to measure, but they make assumptions about how things work so they might be overridden as we learn more. They do get much farther out, over 10 billion light years. Astronomers are using tools like intrinsic luminosity, Cepheid variable stars, type Ia supernovae, gravitationally lensed quasars, gravitational waves, even something called baryon acoustic waves!

Give me my tape measure.

What’s in the Sky?

It’s early riser fun, before sunrise! February 27th look to the southeast and catch Venus, Mars, and the Moon. On March 2nd Saturn and Mercury join Venus and Mars.