A quick (and early) post this week, because Mercury, in particular, is moving fast, and the effect will soon be ruined.

This week, try and get up early.  For some of you I know this is pretty typical, but even those of you that are normally kicking and screaming at 5:30am might have reason to see the pre-dawn sky.

The reason – while Jupiter and Saturn are still dominating the evenings, all the other visible planets – plus the Moon – are occupying one small area of the pre-dawn eastern horizon.

Using tomorrow (Sept 17) as the example and in the figure below, the Moon is a waning crescent.  Below it is the brilliant Morning Star, Venus.  Nearby is Regulus, the star at the bottom of Leo’s “Sickle” asterism, which makes a good handhold as you leap toward the horizon and see Mercury and Mars almost perfectly aligned with each other.

Planetary Lineup at Dawn, September 17.  (via Stellarium)

Mercury is the brighter of the two – it’s almost at it’s 90-degree point from the Sun, running away from us, but extended as far west on its orbit as it will go.  It’s racing back toward (and behind) the Sun a little bit every day.  In fact, get up on a few consecutive mornings and you’ll notice, with Mars as a backdrop, just how fast it’s moving.

Mars, so impressive 6 months ago, is almost diametrically opposed, way over on the far side of the Solar System, as far, and as dim, as it gets.

3D Positions of the Inner Planets, Sept 17, 2017.  (Screenshot from TheSkyLive.com)

This lineup is cool enough, but watch it evolve over the next few days and you’ll get a sense of how dynamic these planets are.  Mercury is racing into the dawn glow and will run away from Mars.  Venus is also sinking lower each morning, and the Moon will be getting thinner and thinner as it passes them up.  Monday it will hang between Venus and Mars.  By Tuesday it will be all but gone.  New Moon is Wed.

In other news this week – say goodbye to summer.  Autumnal Equinox (in the Northern Hemisphere) is Friday the 22nd.  After that, nights are longer than days, and we enter the darkest half of the year.

Finally – it’s not exactly something we can observe in the night sky, but I feel I need to bid a fond farewell to Cassini.  The probe was deliberately crashed into Saturn on Friday, after a 20 year run, and after teaching us some amazing things about Saturn and its moons (particularly Enceladus – which has energy, water, AND organic compounds in the jets that erupt through its icy surface).  Every planetary mission we embark on has expanded our knowledge immensely – and I can’t wait to see what’s next.  Thank you, Cassini!

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