Wild morning display continues with the year’s best look at Mercury


It all happens at 6:30 a.m. Even if you’re up then, will you really go outdoors, or maybe even drive to a place with an unobstructed view towards the sunrise? If you do, you’ll be rewarded: Four planets continue to perform the year’s best conjunctions. The whole thing is bright and easy.

This weekend, for example – Saturday morning, October 17, and Sunday the 18th – the two brightest “stars” hover together. The top and brightest is Venus. The one below is Jupiter. Now look more closely and you’ll see a faint orange star next to Jupiter. That’s Mars. Easy.

All next week, those worlds will keep hanging together, while Jupiter gets higher each morning until it practically touches Venus next weekend. So, really, you can look eastward at 6:30 a.m. any clear morning, whenever you’ve had enough coffee.

Now for the icing on the cake: The three planets just mentioned are not super-low. But if you have, or can get to, a clear shot all the way down – if you live on a farm or a ridge or anywhere where autumn sunrises are visible – you’ll observe a fourth planet. This one’s bright, too, and far beneath the others. This is Mercury, having its best appearance of 2015.

That innermost planet is odd in so many ways that it’s hard to find aspects that aren’t strange. The way it moves, the way it looks and its features all create a carnival of curiosities. Actual radar pulses we bounced off Mercury in the 1960s showed that it has achieved a strange stability with the Sun. It spins once every 59 days, which means that three rotations – three Mercury days – happen in the same exact interval as two of its years. This 3:2 resonance causes Mercury’s sunrises to happen 176 days apart. It’s the longest interval between sunrises of any object in the known universe.

Then, too, Mercury has the most lopsided, out-of-round orbit. So on its surface, the Sun’s intensity varies from six to 14 times ours. When it’s closest to the Sun you want to be sure to use SPF two billion sunblock instead of your usual one billion.

The eccentric orbit also makes that crater-covered planet speed up and slow down more than any other – a variation that would sometimes make a sunrise on Mercury stop in its tracks and reverse itself. The Sun comes up, goes back down, then rises a second time.

Mercury alters its brightness more than any other planet, too, varying by an astonishing 250-fold. It’s best to look for it when it’s on the bright part of its orbit – which is now.

As if jealously to resent Venus’ greater dazzle, Mercury may smash it to pieces. Thanks to gravitational tugs from the Sun and especially Jupiter, the Mercury orbit wildly changes shape. It goes from being a perfect circle to having a squashed eccentricity so great that it may actually reach innocent Venus, the planet whose orbit is closest to being a perfect circle. Such a collision may happen in the next five billion years, and could destroy both planets.

As Mercury spins, it displays not the least axial tilt. Earth, Mars and Saturn are all tilted 20-something degrees, but Mercury alone rotates straight up and down – not even one-tenth of a degree offset from vertical. This means that at its poles, half the solar disk is always below the horizon. Standing within the slightest polar depression or crater at Mercury’s poles, you’d never see the Sun at all. Result: permanent dark places. They’re filled with ice. They offer winter sports on a world badly needing it.

So the next ten days provide the year’s best chance to see that planet with just the naked eye: lowish in the east at 6:30. And as a bonus, you get another three worlds thrown in, for free.


Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at HudsonValleyAlmanacWeekly.com.

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