Good evening all! I have a grab-bag of different topics tonight, some astronomical events, and some catch-up.
First off, Friday was this month’s New Moon. Which means, two lunar cycles (aka months, roughly) until the big Solar Eclipse of 2017. If you haven’t made your plans, it’s not yet too late – but time is running out!
Second, while we’re still talking “Moon” – you know I find occultations interesting, and this week will provide. On Friday, June 30, the Moon will block Gamma Virginis (common name, if you can call it that – “Porrima”), just before 11pm EDT on the east coast. This will be another interesting event because Porrima is bright (the third brightest star in Virgo), easy to find (it’s the one the Moon is about to cover up!), and it is a binary star system – so if you watch closely you can see the star dim, then wink out, as first one star, then the other is blocked. Timetables are here – everybody in the US, EXCEPT the Southwest corner, ought to be able to see this. Folks in San Diego will see the Moon just barely miss.
Third, that sounding rocket launch I talked about two weeks ago? Still hasn’t happened. High winds, clouds, boats in the recovery area. Those poor folks at NASA Wallops have had a real challenge getting this mission accomplished. On the plus side, it means we haven’t missed it yet, though folks on NASA’s Twitter feed are starting to compare it to April, the giraffe.
Fourth, we have two naked-eye gas giants in the evening skies now – so even though it takes longer to get dark, it’s worth staying up. Jupiter is still hanging out near Spica, in Virgo, and Saturn is off to its left, past Antares (the red giant in Scorpio), and technically in Ophiuchus – that “new” zodiac constellation that gave everybody fits last year about screwing up their horoscopes.
This topic is a good excuse for me to catch up on the Zodiac… Everybody knows, or has at least heard of, the Zodiac constellations. They’re the 12 that more-or-less evenly break up the sky into equal parts, align roughly to months of the year, and apparently drive the destinies of every human on Earth. These constellations, and the position of the Sun in them when you were born, are the foundation of your horoscope. Astronomically, they’re important because they lie along the ecliptic – the imaginary giant circle scribed by the average plane of the solar system. The Sun and all the planets lie in this flat disk, and so when we look at the Sun, or at other planets, the backdrop of stars is one of these constellations… only, the ecliptic actually passes through 13 constellations, not 12. There’s some pretty obvious evidence of this right now, as Saturn is clearly hanging out in Ophiuchus (the Snake-Handler). So what’s the deal?
Well, the thing is, constellations – at least to astronomers – are NOT the shapes we see in the sky, and they’re NOT the collections of stars. They’re actually regions, drawn across the sky like territorial boundaries on a map. When we say Saturn has moved out of Saggitarius and into Ophiuchus, as it did some time ago, that’s actually a very precise, discrete moment in time when Saturn crosses that imaginary boundary. The “shapes” – well we typically call them constellations too, but they, like other groupings of stars like the Winter Hexagon, or the Summer Triangle, or the Sickle in Leo, are really “asterisms”.
Even the ancients recognized the constellation that is now Ophiuchus (it had many other names and representations before we settled on an international “standard”), and they also recognized that the constellation area they defined to contain it crossed the ecliptic. But, ancient Babylonians (the ones largely responsible for the signs of the Zodiac, as we know them), were people too – and they ignored Ophiuchus, because it was convenient to do so! Omitting it left a nice, easily divisible set of 12 roughly equal-sized Zodiac constellations. Today, the “signs” of astrology don’t really line up that well with the actual constellations – partly because of Earth’s wobble, or precession, that has caused seasons to shift a bit over the thousands of years since the Babylonians came up with it (and the “sign” calendar was originally correlated to both stars AND Earth’s seasons – Vernal Equinox in particular). So yes, the dates that define our “signs” are off, but astrologists still care about which planets are precisely in which constellations – go figure. The upshot is that the signs are NOT the constellations either – just a set of even, 30-degree arcs of sky that may or may not contain the majority of the constellation it’s named for. Ophiuchus doesn’t count as a “sign”, but Saturn is in it, regardless.
Signs aside, the constellations are still very well-defined regions. If you started with me back in January, we’ve talked about Pisces, Aries, Taurus, Gemini, Cancer, Leo, and Virgo… The next one, Libra, The Scales, is now high in the south after dark, but much as Cancer was defined by its darkness, Libra is really better known as that dark area between Virgo and Scorpio. The balance pans of the scales USED to represent the claws of Scorpio, but now is the only Zodiac constellation representing an inanimate object. It also boasts some of the coolest star names ever given (my opinion, of course) – Zubenelgenubi, Zubeneschamali, Zubenelakrab, owing to its ancient association with Scorpio – “Zubana” means “Scorpion’s Claw” in Arabic.
The next, Scorpio, is one of my favorites, in that it REALLY LOOKS like a Scorpion! I honestly think the depiction in this picture (from Stellarium) ruins it – it looks much better depicted with the claws forming a “T” across the body than this “fan” arrangement. But either way, bright Antares, and the long curving tail of bright stars make Scorpio obvious… Unfortunately, as a summer constellation, it never really rises very high as viewed from the northern hemisphere. (The north is tilted away from the ecliptic during summer nights, forcing the ecliptic closer to the horizon).
Seeing Scorpio is also a hallmark of summer sights though – the heart of the Milky Way is entering the evening sky, the Summer Triangle will soon be high overhead. Lots left to see as we continue our journey around the sun.
Get Out There!
3 thoughts on “Summer Zodiac, etc. (Astronomy: Week of 6/25/17)”
Over the last two nights I finally got a good look at Ophiuchus, as I was trying to see comet 41P/Tuttle–Giacobini–Kresák.
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The comet that just keeps on giving!
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Great stuff. Tonight’s sky was something else. Clear and bright, and that Moon was something.
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