setting Arcturus: "the ghost of the summer Sun"

Event Date: September 7th
Time: 10:00 PM


Brief

   Arcturus is a little over 19º in northern declination.  Although it sets almost four hours after our Sun, that changes by about 2 1/2 minutes a day at this time of year: the Sun sets about 1 1/2 minute(s) earlier each evening as we approach the September equinox, and Arcturus sets slightly less than 4 minutes earlier.  As shown below with a dark sky, we show Arcturus about 2 1/2 hours before it sets.  The celestial equator and grid are present to clarify its declination.  The set Sun is losing declination quickly, now much closer to the celestial equator than Arcturus.

click on image to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

As a result of this gap between sets decreasing quickly between mid August and early October, it seems as if we see Arcturus at about the place in the sky for a few weeks, and then a little lower a few weeks after that.  It isn't until about mid-October that the gap starts to increase again slowly, that we start to have less time to view Arcturus.  With this in mind, along with the Sun being the same declination as Arcturus at Summer's midpoint in late July-early August, there is a coined description for Arcturus: at this time of year, it is called the "ghost of the setting Sun".  After all, Arcturus is orange, which is a little cooler on the temperature scale than our yellow-white Sun.  Also, as a point in the sky instead of a 1/2º disc, it makes us think that the Sun has gone away, and this is what is left as its ghost.

north celestial pole view: now and in 87+ years!

Event Date: September 6th
Time: 9:00 PM


Brief

   Yesterday, I featured Polaris and the north celestial pole (n.c.p.) when talking about the Big Dipper.  Although the Dipper stars of Dubhe and Merak point almost in the direction of Polaris when a line is drawn through both, that same line to the n.c.p. is not quite as direct.  As the first image shows zoomed in with a 1.4º field, Polaris is shown at the edge (circumference) of this field.  Take a look, and the other stars when viewing with a dark sky.  Polaris is precisely 40.9 arc-minutes from the celestial pole...or is it??  See the detailed section for more on that!

Detailed

As hinted above, Polaris is not always the same separation from the n.c.p.  We can recall what I said about precession during the spring, meaning that as all other stars are doing, Polaris is changing in declination slightly each year, over the course of about 26,000 years.  In September 2099 for example, Polaris will reach its maximum declination of 89º 32 527', which according the software, is the highest numerical value.  This will put it less than 28 arc-minutes (1/2º) from the n.c.p., and make those aligning their telescopes most happy!  That is, using it as a guide star in an eyepiece that shows a dim field of stars, it helps recognize patterns closer to Polaris, and getting as close as possible to the n.c.p. before doing any further star aligning; such is common with go-to telescopes.
For the fun of it, I will show the image for September 14th, 2099.  This date and the 28th of that same month of that year, have Polaris at the aforementioned peak-declination, or very close to it both times anyway. 

click on images to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Notice the position difference of Polaris, using the same field as in image one.  For the first one especially, with Polaris barely within the field, can you recognize patterns that will help find the pole?  It is labeled in that image, yet not the second one, since Polaris is closer.  Eyeballing the field in image two however, do you notice which star shown is closest to the n.c.p.?

morning Big Dipper: getting higher

Event Date: September 5th
Time: 6:00 AM

Brief

  Shortly before Sunrise at this time of year, the Big Dipper's "cup" makes a return in the low northeast skies, while its handle becomes more visible with each passing morning.  Depending on how much obstruction there is towards your horizon, we may see the Dipper before Sunrise as early as mid-August.  When looking from a high elevation such as a mountain at mid-northern latitudes, the circumpolar asterism never sets at all.  Because of atmospheric pollution however, it isn't as easy to see when low towards the horizon, so many viewers don't really take notice of it until it starts to stand on its handle. 
Here is the Dipper during twilight, shortly before its stars are washed out by the Sun's light.  The north celestial pole is marked also, to show the Dipper's northern declination.

Detailed

   The more north a viewer is, the more hours (s)he sees the Dipper easily.  From the celestial equator, the Dipper spends about half the day below the horizon.  The further north one goes of mid-latitudes, the longer it can be seen.  Now that we have longer nights as we approach the September equinox, it gives plenty of chances to enjoy viewing the Dipper and--if dark enough with no Moon in the way--dim galaxies with a powerful enough telescope nearby it.  When we start having the Dipper higher in the sky during fall mornings and later Sunsets, I will pick an evening to label those.
This second image is at the same time as the first, although this time, I will take away the landscape, showing only the horizon line and celestial grid.  I will also label two stars of the Dipper--Dubhe and Merak--that point almost directly to Polaris: the brightest star of the Little Dipper, and parent constellation Ursa Minor, being the bear's tail. 

click on images to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

   Dubhe and Merak get some good attention this way; they are often the first stars to learn about when trying to (1) distinguish Polaris from other similar magnitude stars in that part of the sky, and (2) finding as close to north as possible.  Since Polaris is about 40 arc-minutes away from the north celestial pole (labels overlap), it is not precise, yet close enough.  More on Polaris and the north celestial pole tomorrow, when I zoom in on the surrounding star field.

Venus nearing the Beehive...just to pass

Event Date: September 4th
Time: 5:00 AM


Brief

   About an hour and a half before Sunrise, Venus is already high enough in the east.  During the early summer, we saw it near two star clusters in Taurus: the Pleiades and the Hyades.  Both are popular for astronomers, for being visible to the unaided eye, and their appearances that way: the Pleiades often mistaken for the Little Dipper, and the Hyades being a wide V shape.  There is another open cluster that Venus sometimes gets as close to as we saw it get to the Hyades, and it is approaching that cluster over the next several days: the Beehive.  Shown below, we find that being only 1º 20' from the ecliptic makes the Beehive a regular target for planets such as Venus and Mercury; both have constantly moving orbits on either side of the ecliptic--seen from Earth anyway! 

click on image to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

At magnitude 4.0, the Beehive requires a much darker sky to see; with such, the cluster looks more like a smudge.  If using binoculars or a low-powered telescope, the Beehive's many, many stars resemble "bees swarming"; in this case, near or around a hive.  If the timing is right, the planet can go very close to--if not directly through the center of--the cluster.  For this to happen, we need to be in a position looking towards Venus when its orbit puts it in line with the cluster the same time that we see the planet go in front of it.  There are times when the orbit is perfectly placed but the planet is still a few--if not several--degrees away; that is the case this time, meaning that Venus and the cluster's closest encounter in a little over a week will have them paired well in a wide-field telescope, yet not showing Venus occulting any bees...er, stars!

both triangles--morning sky panorama

Event Date: September 3rd
Time: 4:17 AM

Brief

We are 5 mornings away from a very fine sight.  That is, for most of August, I have talked about the "2nd/other triangle" in the east, while our main Summer Triangle of (V)ega/(D)eneb/(A)ltair. sets earlier each morning in the northwest.  (V)enus/(J)upiter/(S)irius have been changing triangular configuration daily as Venus has separated from Jupiter.  Although Jupiter has changed separation from Sirius, it has been much slower than Venus, which moves much faster than prograde than Jupiter.  Getting back to the aforementioned 5-morning forecast, this V/J/S triangle will look more and more isosceles.  It is larger than the size as V/D/A, with current separations of 39º, 42º, 47º (V/S, V/J, J/S respectively).
   Below is a 180º full-zoomout of the sky, to display both triangles.  This interesting feature of the software lets us see the triangles at the same time; our normal vision of 100º doesn't let us do this normally.

click on image to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

   The V/D/A stars have separations only as large as 38º (V/A).  If you have looked in the sky at V/D/A and imagine it about 25% larger, that is about the size of V/J/S.  Also, since Altair now is low in atmospheric pollution as early as 4 AM, Sirius is just starting to rise at this time.  For how many minutes can you see all 6 stars while Sirius and Altair are low?  Vega is about as low as Altair, yet sets a little later being further north.  At the time above, Altair and Sirius are about the same altitude.  Now that they are rising before twilight, both may be easy enough to see with binoculars, even with heavy atmospheric pollution.
   Keep watching V/J/S, as Venus continues to pull away from both slightly.  Can you see how two "sides" are starting to look the same "length" as a result of the separations?  Look closely, and use your fist stretched out at arm's length if that helps measure them.  Remember that a fist is about 10º held out this way, with each knuckle about 2º.

Mars pulls further from Saturn, Spica

Event Date: September 2nd
Time: 8:00 PM


Brief

  Here is a quick look at the Mars/Saturn/Spica grouping...or what was!  Mars has continued its separation from the other two, while much slower moving Saturn, has stayed close enough to Spica to easily fit in a binocular field with the star: just under 5º separated.  Since we have had the Sun setting more than a minute earlier each day the last several days (and continuing at that rate on average for this month), it has meant that we have been able to see the trio before they get too low to the horizon.  Here they are, with Mars already separated from Saturn by 10º.  Less than three weeks ago, they were in a nearly straight line, which just goes to show that as we move further from Mars in our orbit, that we see it moving against the stars faster as well.

click on image to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

   Although we are further from Mars each day, we don't see it change it phase much as we do with the inner planets; there is some wax and wane as our position with Mars changes, although it is minimal compared to the the crescent phases that we see of Mercury and Venus.  With Mars, when it starts to approach the other side of the Sun, we still have a lit side facing us, as it only has superior conjunction and not inferior.  With the latter, in the case of Mercury and Venus, we are seeing the shadowed side of each, opposite the Sun as they go between us.  However, as we see with Mars and less so with each outer planet the further we go through Neptune, a hypothetical viewer on each of the inner planets facing Earth would see a certain percentage of wax and wane throughout the viewing of Earth.  Since we will never find ourselves on either planet, I will hold off from showing images from that, yet it is a very similar concept as from Earth viewing Venus and Mercury.  If viewing Saturn through Neptune from here, the change in phase is minimal enough that most people don't even think to look for such.

Sun transits getting lower, entering September

Event Date: September 1st
Time: 1:08(.31) PM


Brief

As we approach these last three weeks of summer, it is a time that we see the Sun move further towards the southern declinations as fast as we see all year.  When looking at the image below, featuring the Sun, celestial equator and ecliptic, we see a decrease in altitude for transit happening each day.  By looking left (east) of the Sun, the green ecliptic is seen coming towards the celestial equator and dipping below it.  As the Sun follows this path, it dips more than 11º during the last month of summer, and that same number for the first month following the September equinox.  I showed a similar scenario shortly after the March equinox, as the Sun spent the following 4 weeks becoming further north in declination, albeit slightly slower each day during that span.
  The image here shown here displays the Sun at transit.  The celestial guidelines mentioned, as well as the meridian, all show.

click on image to enlarge: courtesy of Starry Night Pro Plus, version 6.4.3, by Simulation Curriculum Corp.

Detailed

   As the Sun gradually gets lower at transit, it of course, spends less time above the horizon,  By winter, especially the first few days, our star is only 29º above the horizon, or  just shy of 1/3rd the way between the it and zenith.  Right now, at about 8º north, the Sun transits just over 60º in altitude, or 2/3rds of the way between horizon and the sky,  Although transit altitude is based on latitude, one thing is for sure this month: the altitude will always be the same number of degrees for the celestial equator.  Therefore, for viewers at our global equator, the Sun transits at zenith at equinoxes.  For viewers between there and 23 1/2º north and south of the equator, they also have zenith transits on other days of the year, depending on their exact latitude.  For example, someone living at 11º north, somewhere in Central America, will have a zenith transit of the Sun around April 18-20st (depending on whether or not it is a leap year), and also around August 19-21st.