• Choose a month

  • 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.)

Look east In September 2013 – and take a journey from mythology to science

As we travel September skies we’ll move from the age of mythology to the age of science.

First, the age of mythology. Had you been born a few hundred – or even a few thousand – years ago, the eastern sky in September shortly after sunset would look something like this to your imaginative eye.

For most of recorded human history different cultures turned the stars into familiar patterns that  illustrated familiar mythological stories. In our September eastern skies shortly after sunset we have a wodnerful collection of five related mythological figures - Cepeheus (king), Cassiopeia (queen), Andromeda (princess), Perseus (hero), and Pegasus, the flying horse. (Developed froma screen shot of  Starry Night Pro. Click for larger version.)

For most of recorded human history different cultures turned the stars into familiar patterns that illustrated familiar mythological stories. In our September eastern skies shortly after sunset we have a wonderful collection of five related mythological figures – Cepheus  (king), Cassiopeia  (queen), Andromeda  (princess), Perseus  (hero), and Pegasus, the flying horse. (Slightly modified screen shot of Starry Night Pro. Click for larger version.)

The tale is easy to remember. The king (Cepheus) and queen (Cassiopeia) felt their kingdom was threatened by a sea monster, so as a sacrifice to the monster they tied their daughter, Andromeda, to a coastal rock. m But don’t worry, our hero Perseus, fresh from slaying Medusa, appears to rescue Andromeda, and they ride off across the starry heavens on his faithful steed, Pegasus, the flying horse.  Really – today the king and queen  would be tried for child abuse!

Of course as usual with the ancient constellations, the figures bear only the crudest relationship to the pattern of bright stars, so a lot of imagination is required to see them.  But that said, I do find this myth an easy way to remember these five constellations. In modern times we’ve drawn complex boundaries around each constellation and used these imaginary celestial boundaries to name and locate stars.  But more importantly, we’ve developed a celestial coordinates system much along the lines of Earthly longitude and latitude.

If you imagine the Earth’s latitude lines projected onto the dome of the sky, they become circles indicating declination – how far in degrees a point is from the celestial equator. The celestial equator itself is a projection onto the sky dome of Earth’s equator.  Longitude is projected and marked in 24 hours of “right ascension” so the whole celestial clock appears to pass overhead in the course of a day. I found it difficult remembering where these hours begin until I learned about the “Three Guides.” These are three bright stars –  indicated by arrows in the chart below – that fall very close to the zero hour line of right ascension. Above them (think of these as preceding them, for they rise first)  the hours count backward from 24. Below them – think of these  as following them, since they rise afterwards – are the hours counting up from 0.

Prepared from a Starry Nights Pro screen shot - click for a larger version.

I prefer to remember my sky in terms of bright stars and asterisms, so Cassiopeia becomes the “W.” Andromeda  becomes “Andromeda’s Couch,”  and the flying horse becomes the “Great Square.”   But the Three Guides – the three bright stars indicated by arrows – allow you to see the sky in more scientific terms, for these are the starting points for laying a grid on the sky to create a precise address for each star in terms of its places on the grid. This grid is indicated above by the red lines. (Prepared from a Starry Nights Pro screen shot – click for a larger version.)

For a printer-friendly version of this chart, download this.

First, let’s look at the “Great Square” – or perhaps we should say “Great Diamond,” since that’s what it looks like when rising. Once overhead, it is certainly a square, and it forms the heart of Pegasus – the flying horse. The stars are all second and third magnitude – about the brightness of the stars in the Big Dipper – so wait until about an hour after sunset, then look east and you should be able to pick this out. Its stars mark out a huge chunk of sky that is nearly empty of naked-eye stars, which is why I sometimes call it the “Great Empty Square.”

Andromeda’s Couch, ties to the northern corner of the square. In fact, it shares a star with this corner. “Andromeda’s Couch” is just my memory device – others would simply call this “Andromeda” because that’s the name of the constellation it dominates. I have difficulty seeing the lovely maiden chained to a rock by looking at these stars.  But knowing that in myth Andromeda was a lovely woman who was rescued by Perseus, I like to think of this graceful arc of stars as her couch with her a misty fantasy figure lying there in alluring fashion. That said, notice three things about it:

1. The bright star at the right – southern – end is also a corner of the Great Square, as we mentioned. In fact, it is the brightest star in the Great Square.

2. The three brightest stars in the “couch” – I’m ignoring the second star which is fainter – the three brightest are about as close to being identical in brightness as you can get – magnitude 2.06, 2.06, and 2.09. They also are pretty equally spaced. Hold your fist at arm’s length and it should easily fit in the gaps between these stars, which means there are 10-15 degrees between each star. That’s similar to the spacing between the four stars in the “Great Square” as well.

3. The second star, as mentioned, is dimmer by more than a full magnitude (3.25), but it’s what gives this asterism a couch feeling to me – or maybe a lounge chair – marking a sharp, upward bend.

And where’s the hero Perseus? he should be nearby, right? Well he’s on his way, rising in the northeast after Cassiopeia, but we’ll leave him for next month when he’s more easily seen.

Now for the pièce de résistance!

This is a group of stars that are new to me, at least in this role, and I love them! They’re called “The Three Guides,” but I think of it as four guides They can all be tied together by a long, graceful arc that represents the great circle of zero hour right ascension – which is the “celetsial meridian” as defined in the equatorial coordinate system.

As mentioned, the equatorial coordinate system is essentially a projection of the Earth’s latitude and longitude system onto the sky to enable us to give a very precise address for any star or other celestial object, as seen from our planet. On Earth we require an arbitrary circle be chosen as the zero longitude line, and this is the circle that passes through the poles and Greenwich, England.

In the heavens we also need such a circle, and the one chosen is the one that passes through the point where the Sun crosses the celestial equator at the vernal equinox. But that point is not represented by any bright star, so how do we know where this “zero hour” circle is? We need it to put numbers to the entire system. Enter “The Three Guides.”

They start with the star Beta Cassiopeia. This is the western most star in the familiar “W”  – the one which rises first and leads the rest. (Remember – all stars appear to move westward as the earth turns.)  From there draw an arc to Alpha Andromedae. This is the star mentioned before where Andromeda and the Great Square are joined – they both share this star.

The third star of this trio is Gamma Pegasi – the star that appears to be at the bottom of the Great Square when we see it as a diamond when rising. (If this is not clear, one glance at the accompanying chart should make it so.)

When I look at this great arc, however, I always start to trace it right from the North Star, Polaris. All the great circles representing meridians of right ascension pass through the north and south celestial poles.

As you move upward from this zero line in the general direction of the Summer triangle, the hours count backwards counting the Zero Hour as 24. Move downward, towards the horizon and the hours count forward from zero. This sequence is marked on our chart around Polaris.

Taking a wide view of the “Three Guides” to incorporate the North Star and Summer Triangle as well. Here’s what we should see about an hour after sunset. Click image for larger version. (Derived from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, download this.

What’s important is to be able to visualize this one circle in the sky and connect it with the another circle crossing it at a right angle – the celestial equator. If you can do that, you will have identified the two zero points on the equatorial coordinate system and moved your knowledge of finding things in the sky from the mythological arena to the scientific one. That’s why these three “guides” excite me so. When you can look up at the night sky and see not only a dome, but a curved grid projected on it, and on this grid be able to attach meaningful numbers, then you have graduated to sky explorer, first class!

. . . and the rest of the guideposts?

If you’ve located the new September asterisms and identified The Three Guides, then it’s time to check for the more familiar stars and asterisms you might already know, assuming you have been studying the sky month by month. (If this is your first month, you can skip this section.) So here are the guidepost stars and asterisms still visible in our September skies.

  • The Summer Triangle is now high overhead, though still favoring the east. Vega, its brightest member, reaches its highest point about an hour after sunset and moves into the western sky. Altair and Deneb are still a bit east, but will cross the meridian within about three hours of sunset.
  • The “Teapot,” marking the area of the Milky Way approaching the center of our galaxy, is due south about an hour after sunset. Well into the southwest you’ll find the red star Antares that marks the heart of the Scorpion.
  • Arcturus (remember, follow the arc of the Big Dipper’s handle to Arcturus) is due west and about 25 degrees above the horizon as twilight ends.
  • The Keystone of Hercules and the circlet that marks the Northern Crown can both be found high in the western sky by tracing a line between Vega and Arcturus.

Look north in September 2013 – the king’s on the rise!

Yes, that’s Cepheus, the King – remember that Cassiopeia (the “W” ) is the Queen. Though Cepheus makes a familiar “home plate” asterism, it’s not nearly so memorable as the “W” of Cassiopeia, primarily because its stars are dimmer than those of the “W.” In fact, you might have difficulty picking it out at first, but here’s a tip: Follow the familiar “Pointers” of the Big Dipper to the North Star – then keep going. The first bright star you meet will mark the tip of the Cepheus home plate. (It’s about one fist away from Polaris – the Pointer stars are nearly three times that far in the other direction.)

Also coming up below the “W” is the “Bow” asterism that marks Perseus, who is carrying the head of Medusa, which contains the “Demon Star,” Algol. We’ll take that up next month when they’re higher in the sky and easier for all to see. Here’s a chart.

Click image for a larger version. (Developed from Starry Nights Pro screenshot.)

For a printer-friendly version of this chart, download this.

To review the connecting mythology, which helps me remember the related constellations, here’s the story in brief.

Cepheus and Cassiopeia have a daughter Andromeda whose beauty makes the sea nymphs jealous. They enlist Poseidon to send a sea monster to ravage the coastline of Ethiopia, the kingdom of Cepheus and Cassiopeia. To appease the monster, the good king and queen chain Andromeda to a rock along the coast, but Perseus rescues her and together they escape on Pegasus, his flying horse.

You meet Andromeda and Pegasus – the flying horse is much easier to identify as the “Great Square” – in the “look east” post this month. Also in the “Look East”  post we detail the “Three Guides,” three stars that mark the zero hour in the equatorial coordinate system used to give a permanent address to all stars. The first of those Three Guides is Beta Cassiopeia, visible in our northeastern sky, and so on the chart with this post.

Moving from mythology to science, Cepheus is probably best known today for a special type of star called a Cepheid variable. This is a star that changes in brightness according to a very precise time table. What’s more, it was discovered that the length of a Cepheid’s cycle – that is the amount of time it takes to grow dim and then brighten again – is directly related to its absolute magnitude. The absolute magnitude of a star is a measure of how bright it really is as opposed to how bright it appears to us. (How bright it appears is, of course, related to how far away it is.) That makes Cepheid variables a sort of Rosetta Stone of the skies.

It is relatively easy to time the cycle of a variable, even if the star is quite faint from our viewpoint. These cycles usually cover a few days. If you can identify the length of this cycle, you then can know the absolute magnitude of a star. And if you know its absolute magnitude, it’s a simple matter to compare that to how bright it appears to us and thus determine its approximate distance from us.

This is a huge breakthrough. Without Cepheid variables astronomers were at a loss for determining the distance of anything that was more than a few hundred light years away. The distance to such “close” stars could be determined using a very common method known as parallax – that is, determining how the star appeared to change position slightly from opposite sides of the Earth’s orbit. But that change in position is extremely tiny and difficult to measure even with very close stars. With the Hipparcos satellite and computer analysis, it has been possible to use this parallax system for stars as far as 3,000 light years. But that still is close by astronomy standards. (Keep in mind our galaxy is about 100,000 light years across.) But Cepheid variables can even be found in other galaxies. In fact, they played a huge role in proving that “spiral nebulae” were really other “island universes” – that is, other galaxies. The Hubble Space Telescope has found Cepheids out to a distance of about 100 million light years – a huge leap from the 3,000 light years we can reach with the parallax method.

There are other ways of making an educated guess at an object’s distance, and they frequently are quite complex and indirect. But the Cepheid variable has been one of the most important tools in the astronomer’s tool kit for the past century. It was in 1908 that Henrietta Swan Leavitt, a $10.50 a week “calculator” at Harvard Observatory noticed a pattern while doing tedious work cataloging stars and saw it’s importance. Though she published a paper about it, she never really received the credit she deserved during her lifetime for this breakthrough discovery.

So when you look at this “home plate” in the sky, see if you can find the fourth magnitude star, Delta Cephei – it’s not hard to spot under good conditions. (See the chart above.) When you find it, pay homage to it for the key role it has played in unlocking the secrets of the universe – for once astronomers know the distance of an object they can make all sorts of deductions about its composition, mass, and movement.

Mars will NOT be close to Earth, nor “as large as the Moon” in August – this year or ever!

Forget what you might have read in an email, or on Facebook, or whatever – this is the truth about how we see Mars on those every-other-year occasions when we see it at its best!   This graphic was created  by Tom Harradine and highlights the  simple reality – Mars will never be as big or as bright as the Full Moon, or even a crescent Moon.


Click the graphic to get a larger version.

Sorry I have to write this, but every August since 2003 I have gotten questions about a spectacular showing of Mars in our sky because an anonymous email   or social media post makes the rounds of the Internet causing people to get excited. THIS EMAIL IS NOT TRUE. I wrote about it other years pn this site. Here’s my updated version.

The most outrageous claim in this email goes something like this:

On the night of Aug. 27, the planet Mars will come closer to Earth than it has in the past 60,000 years, thereby offering spectacular views of the Red Planet.  Mars will appear to the naked eye as bright as and as large as the full moon. No one living today will ever see this again!

No one living today will ever see this. Period. OK. here’s the simple truth.  An hour before dawn  August 27, 2013 Mars will be rising in the east and be about two fists – 20 degrees – above the horizon. It will shine a little dimmer than  two nearby stars, Castor and Pollux. As always it will have a reddish tinge to it, but to the naked eye look just like any bright star. 

In a very good telescope, Mars will look like a very tiny planet, about one-fifth the size it is when it actually does make a relatively close approach to Earth.  Mars and the Earth are relatively  close to one another  – 35 to 60 million miles apart – every two years.  In AUgust 2013 Mars is about 214 million miles away, or bout 6 times as far away as it it is on closest approach and this appearing – telescopes – about one sixth the size it appears on closest approach.   (Mar’s at it’s absolute closest is still – in telescopes – only about half the size of Jupiter.

Is there any truth in the email? Yes, there are some grains of truth here, but even they are usually distorted. This all began because in August 2003 Mars really did make an unusually close approach to Earth – but by unusual we mean just a bit closer than it gets routinely every two years.  During a close approach such as the one in 2003 Mars can be 24 seconds of arc across – and sometimes it’s “close” approach means its half that size.

In short, this Mars email is just another Internet urban legend telling us things that would be fun if they were true, but really are far, far from the truth. For details on this, please visit the Snopes.com site – in fact, anytime you receive an email that sounds too good to be true – on almost any subject – check snopes.com before you forward the email to friends. It’s a great clearing house for getting out the truth on these legends.

%d bloggers like this: