We have a pretty conjunction on tap for late tonight when the waning gibbous moon shines just 1.7 below brilliant Mars. If you have a clear view down to the eastern horizon you'll see the bright pair as early as 11:15 p.m. Most of us will have to wait until around midnight when the duo rises high enough to clear local obstructions like trees and buildings. Or you can take a peek out the window at dawn tomorrow morning when they'll stand high in the southern sky.
Mars keeps getting brighter and brighter. At magnitude –1.3 it's nearly the equal of Sirius, which at –1.46 is the brightest of the nighttime stars. Magnitudes are a way to rank stars according to brightness and goes back to 129 B.C. when the Greek astronomer Hipparchus invented the system we still use today.
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Hipparchus assigned 1st magnitude to thebrightest stars, 2nd magnitude to the next brightest all the way down to 6th magnitude, the faintest star visible with the naked eye. He didn't know it at the time, but each step corresponds to a change of 2.5 times in brightness, so a 1st magnitude star or planet is 2.5 times brighter than a 2nd magnitude one which in turn is 2.5 times brighter than a 3rd magnitude star and so forth.
His system worked well but lacked subtlety because all stars in a particular magnitude rank are not equally bright. Even a quick glance at Vega and Deneb — both 1st magnitude stars in his catalog — will reveal that Vega is obviously brighter. When the telescope was invented in the early 1600s thousands of new stars fainter than sixth magnitude were discovered.
To accommodate these new realities astronomers expanded the magnitude scale both forwards and backwards. Vega proved to be a full magnitude brighter than the average first magnitude star, so with nowhere to go but backwards Vega's brightness was refined to magnitude 0. Sirius is brighter yet, putting it squarely in negative number territory at –1.5. Early telescopes could reach 7th and 8th magnitude, so the scale expanded forward to include them. That expansion continues to this day — Hubble can detect objects down to magnitude 31!
Not only is Mars bright, but its apparent size is growing. If you have a telescope and haven't looked at the Red Planet up close yet, start now. Click on the Mars Profiler to find out which hemisphere faces your way any time of day or night. It will help you identify the dark markings visible on the planet's surface. Did you know that Mars is the only planet we can see through amateur telescopes that has easily visible surface features? All the rest are covered in clouds with the exception of Mercury, but it's so small and relatively far away that it's exceedingly difficult to see anything but its phase.
Clear skies and enjoy the conjunction!
