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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

September 2013 – pursuing the not-so-false dawn – plus planets

There’s nothing false about the false dawn – in fact, it’s quite intriguing and somewhat puzzling, but very real. Here’s a cool picture of it.

Yes, I’m talking about the zodiacal light – known for hundreds, if not thousands of years as the “false dawn” because it precedes the usual predawn light. Only it isn’t always so obvious – September and October are the best time to see it in the northern hemisphere early morning sky.  (It is best seen in the early evening sky in February and March.)

Oh  – do keep in mind that the picture above was taken through the thin air and superbly dark skies above the European Southern Observatory in Chile and  benefits from the camera’s ability to do a better job of capturing faint light  than our eyes.  We won’t see it that way. But,  the picture is very useful because it gives us a good idea of the shape and size of what we are looking for when we seek this elusive glow in our skies.

If you want to catch it you have to:

  • Be out two hours before sunrise – and  give your eyes time to dark adapt. It is best seen about 80 minutes  before sunrise.
  • Be in a place relatively free of light pollution – you especially don’t want to be looking at a light dome from a city to your east. If you can see the Milky Way your skies are dark enough – if not, you need to go somewhere where you can see it.
  • Look at a time when the Moon isn’t in the morning sky – in 2013 that means the first two weeks of either September or October. 

Is it worth it – I certainly think so – but then I think September mornings are great anyways because you get to see all the bright stars of the Winter Hexagon without freezing your tail off as you do when they are in the evening sky in January. In addition we have Mars rising low in the East and Jupiter is already pretty high up and should appear near the peak of the zodiacal light – and be brighter than any star.  (Mars will be about as bright as   Castor and Pollux,  two of the bright stars  of the Winter Hexagon. Here’s a chart.

Click for larger image. Prepared from a Starry Nights Pro screen shot.

Click for larger image. Prepared from a Starry Nights Pro screen shot.

Click here for a printable, black and white version of this chart.

For you insomniacs – or folks who just love to get up early when the world is still and most of the neighbors have turned off their lights so the sky is darker, pursuing the zodiacal light is special.

What is it? It is sun light reflecting off a  huge cloud of very fine dust between the Earth and Sun on the plane of the solar system.  That’s been agreed upon for some time.  How much dust?  Well, wrap your mind around this.  Assuming that the dust particles have the same reflectivity as the surface of the moon, it would take one dust particle every five miles to reflect that much light! We’re still looking at an awful lot of empty space. Hmmm. . . there 93 million mile between the Earth and Sun – so if we had a single straight line of dust particles, we’d still have more than 18 million of them – and of course this is much more than one single line.  Now that’s awesome.

But where did all that dust come from?    J. Kelly Beatty goes over the science history in an excellent article in September’s Sky and Telescope  and notes that the current opinion is the dust cloud is a result of short period comets.  Think of a comet as a dirty snowball that melts as it nears the Sun, leaving a trail of dust. That dust stays in orbit. Short period comets are ones whose orbit takes 200 years or less because they have been captured by the gravity of the  planets. (Other comets take much longer to orbit, or simply make a single trip around the Sun.)

Why is it obvious in the morning sky in the fall and the  evening sky in the spring? Because it follows the path of the ecliptic and the ecliptic is more or less straight up and down in the morning at this time of year – and in the evening in February and March. At other times it slants at quite an angle keeping the zodiacal light lower in the sky where it gets lost in the routine dawn light.

There’s a nice little planetary show in the west this month as well, as Venus and Saturn get cozy and on September 8 Venus has a close encounter with the crescent moon right after sunset.   It’s Saturn’s turn the next night.  About a week later  Saturn and Venus should fit comfortably in the same binocular field of view for several days.

Click for a larger image. (Prepared from a Starry Nights Pro screen shot.)

Click for a larger image. (Prepared from a Starry Nights Pro screen shot.)

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