By Mike Lynch
I urge you this week to get out in the very early evening, about 45 minutes after sunset, to see the celestial hugging between Jupiter, Venus and Mercury.
Look for the three planets in a tight little triangle just above the horizon.
You have to have an unobstructed treeless view of the west-northwest horizon, but you don’t need a telescope.
They’re within 2 degrees of each other, so close that your thumb extended at arm’s length can pretty much cover up all three worlds. They will slip below the horizon by about 10 p.m.
Venus is the brightest of the three and Jupiter is the farthest away at more than 560 million miles.
The triangle will break up as the week goes on, but all three planets will still be hanging close together. And these three are closer to us than any other stars.
Stellar distances are too cumbersome to express in miles. Light-years do a better job because the numbers are smaller and you’re reminded of just how long it takes for the light from the stars to reach your eyes.
Light travels at the speed of 186,300 miles a second and a light-year is defined as the distance that light travels at that speed in one year.
Given that there’s about 31.5 million seconds in a year, you’ll come up with almost 6 trillion miles for just one light-year.
So if a star is 100 light-years away that star would be about 600 trillion miles away. That also means the light you see from that star took about 100 years to reach your eyes.
How do astronomers know how far away these stars are?
For stars within about 3,000 light-years from Earth, astronomers use the stellar parallax method for determining distance.
Basically a picture of a star is taken when the Earth is on one side of the sun in its orbit, and another picture is taken six months later when the Earth is on the other side of the sun.
If the star is not too distant, you’ll see it shift a tiny bit against the background stars. The shifting of the star against the background stars creates what’s called a parallax angle.
You can calculate a star’s distance using simple geometry that states opposite angles are equal plus some simple trigonometry.
The practice of measuring that parallax angle is very difficult and you’re also making assumptions: You’re assuming that the background stars you are using to measure the angle are stationary. In reality they may be shifting as well.
The Hipparcos satellite was launched in 1989 to measure the stellar parallax and distances to hundreds of stars, but the satellite’s accuracy falls off with smaller parallax angles and larger stellar distances past about 500 light-years.
Stars beyond that require another method, the Hertzsprung-Russell diagram, developed in the early 1900s by Ejnar Hertzsprung of Holland and Henry Norris Russell of the United States.
For really distant stars, Cephied variable stars are used, a huge discovery made by Henrietta Leavitt early in the last century at Harvard University.
The famous astronomer Edwin Hubble used observations of Cepheid variable stars in what was then known as the Andromeda Nebulae to determine that Andromeda was a whole other galaxy, more than 2 million light-years away. Until then, our Milky Way was thought to be the only galaxy in the universe.
Mike Lynch is an astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and is author of “Stars, a Month by Month Tour of the Constellations.” www.lynchandthestars.com.
The Everett Astronomical Society: www.everettastro.org/.