Hey, sky fans. Thanks for spending some of your Saturday night here. I’ve got the bottle of Traquair House Ale, and the “Meatballs” on the ol’ DVD machine.
Tonight’s full moon, May’s, is called the Flower Moon because of the… you know… the… the flowers, which were brought by the April showers. You also may have read that it’s a seasonal blue Moon. Most people these days think of a blue Moon as the second full Moon of month. A seasonal blue Moon is the third of four in a season. Either way, remember, it’s not actually going to be blue. If you’re a fan of blue moons of any flavor, make sure you soak it up tonight. The next monthly blue Moon is a ways away, at the end of January 2018.
Well, it’s raining here, but hopefully the skies are better where you are, and you can see tonight’s meeting of Mars, Saturn, which is just a couple fingers-at-arm’s-length-away, the Flower Moon, and the bright star Antares in the southeast sky. To remind you, this clique will be at its highest when it reaches due south at around 1am Sunday morning, but it should be visible all night once they get above the buildings, mountains, elephants, trees, or ballparks that block the low part of the sky.
If you can see it, you might have noticed that Mars is really bright, kind of alarmingly bright, for such a small planet—it’s the second smallest major planet in the solar system, only about 4,200 miles across, which is a bit more than half the diameter of the Earth, and it’s about 50 million miles away. Tonight, Mars is at opposition, which means the Sun, Earth, and Mars are in a straight line. This arrangement can happen anywhere along Mars’s and Earth’s orbits as long as the line between the two and the Sun is straight. This happens about every 16 months. These days, Mars is as bright as its been since 2003.
Since the line is straight, Mars’s face is fully lit as we see it from Earth. If this sounds familiar, this is the same arrangement as when the Moon is full: The Sun, Earth and Moon are in a straight line. So, by a terrific coincidence, both Mars and the Moon are at opposition tonight. Yep, there’s lots of big rocks out there reflecting a lot of sunlight straight back at us. When that line is off to one side or the other, less of the face is lit, and the Sun’s light isn’t reflected back as directly, just like staring straight at a flashlight is brighter than off to the side a bit. This, this being off to one side or the other, is what we see as the Moon’s phases.
Interestingly, even though Mars is at opposition, it’s going to keep getting brighter for another few days. The orbits of the planets aren’t circles, they’re ellipses. Their orbits aren’t perfectly centered around the Sun, and the these eccentricities aren’t in the same places relative to each other. Nothing’s concentric. At the moment, the Earth is moving farther from the Sun in its orbit, which it will keep doing until it reaches aphelion – it’s farthest point – in early July. Meanwhile, Mars, in its orbit, is moving closer to the Sun. This means the Earth and Mars are moving closer to each other – Earth outward from the Sun; Mars inward. Mars’ll reach its closest point to the Sun, it’s perihelion, on May 28th. It and the Earth will be at their closest point to each other, their closest approach, for this trip around the block on the 30th and 31st. It’s close, but this isn’t quite their perigee. That’s their nearest possible points to each other. This time, close but no cigar.
Have a look at these very-not-to-scale sketches I made. On the left, you’ll see tonight’s opposition, when the Sun-Earth-Mars line is straight. On the right, is next week’s closest approach, when the line isn’t straight but the orbits are closer. Hopefully, they’ll give you a better idea of what I’m going on about.
_____ skies, everyone!