Sunday, January 17, 2016

A procession of the gods

If you make it a habit to go outside to gaze into the night sky -- and I strongly encourage everyone to make it a habit, if it is at all possible to do so -- you will no doubt have been enjoying the glorious spectacle of Orion, huge and very visible at this time of year, rising up towards the highest part of his arc across the heavens during the "prime-time" viewing hours between sundown and midnight.

Due to the progress of the earth's journey along its orbit around the sun throughout the year, Orion rises a few minutes earlier each night (about four minutes earlier), causing him to be further and further across the sky on successive nights (not much further each night, but even in the course of just one week he will be noticeably further along).

This nightly western progress means that new constellations begin to rotate into view each night, rising up a bit earlier above the eastern horizon on each successive night -- and for the past few weeks, the majestic form of Leo the Lion has been looming up above the eastern horizon during those "prime time" star-gazing hours, getting higher and higher each evening.

This means that if you step outside at 10:00 each evening (or 22:00 if you are using the 24-hour method), Leo will be a bit higher each night that you step outside at 10. And, as Leo gets higher in the sky, if you have been going outside around 10:30 or certainly 11 pm, you cannot have failed to notice the brilliant orb of an extremely bright planet rising just below the stars of the celestial Lion (visible by 11 pm for most readers, even if you have a fairly high horizon skyline when you face to the east).

Now that we have access to the web at all times, it is pretty easy to look up any of a number of websites to tell you the name of the planet whose golden beams of reflected light you are enjoying in the east in the vicinity of Leo. But, if you had lived thousands of years ago, prior to the invention of networked information-bearing devices, how might you have identified this planet?

First, as you become more familiar with the night sky (which will happen over time, especially if you make it a habit to go out at about the same time each night, every night or nearly every night), you would immediately realize that this glowing light is not always visible in Leo, and hence it is not one of the "fixed stars." Thus, you would realize that it is a planet, a term whose name means "wanderer," because the planets travel through the different constellations (always roughly along the same ecliptic path that the sun and moon travel, and hence always through the constellations in the zodiac band).

Second, as you become more familiar with the five visible planets, you would realize that this glowing sphere presently seen below Leo (it's actually at the top of Virgo now, following just behind and a little below Leo in the turning zodiac band) must be either Jupiter or Venus, just by its brightness and its brilliant golden light.

The warm golden glow of this planet is not characteristic of either Mars or Saturn, both of which are dimmer when seen from our planet's surface, and give off a slightly different color and feeling (both are considered "less benevolent" than Jupiter or Venus). Mars, of course, gives off a reddish light when the sun's rays reflect back to us from its surface, and Saturn gives off a darker yellow tone. Mercury gives off an somewhat deep-orange hue, but is very small and difficult to spot and cannot possibly be mistaken for the planet we're discussing near Leo either.

Thus, knowing that this planet must be either Jupiter or Venus, you would have to know something about the orbital paths of our solar system's planets in order to determine which one it could be. Because this planet is rising (in the east, where all celestial bodies including the sun are seen to rise, due to the fact that our earth rotates towards the east) so late in the evening, long after the sun has disappeared below the western horizon, we can conclude that it must be Jupiter and cannot be Venus. 

The reason for this is fairly simple, if you think about the fact that Venus follows an orbital track that is interior to the earth's orbit, relative to the sun: the path Venus follows stays within the near-circle of the earth's path at all times. Thus, in order to see Venus, we will always have to look generally towards the sun, although when the sun is up it will be extremely difficult (and usually impossible) to see Venus, so it is best to observe Venus just before the sun rises or just after it sets. 

Venus will be visible above the western horizon after the sun goes down, if Venus is at a point on its orbit that causes it to "trail" the sun from the perspective of an observer on earth. Venus will be visible above the eastern horizon before the sun comes up, if Venus is at a point on its orbit that causes it to "lead" the sun from the perspective of an observer on earth. More detail about where Venus would have to be in its orbit around the sun in order to appear to be either "trailing" or "leading" the sun can be found in this previous post

But the main point is that, due to the fact that its interior orbit will always make it so that we have to look generally in the direction of the sun in order to see Venus, the planet appears to be "tethered" to the sun (albeit by a fairly long "tether"), and thus Venus will always be seen above the same horizon that the sun is also about to "rise out of," or above the same horizon that the sun just sank beneath.  Venus will never be seen to rise above the opposite horizon from the sun. 

Above is a star chart showing the night sky looking due south, from the perspective of an observer in the northern hemisphere at about 35N latitude. Some of the brightest constellations and celestial features are outlined for you, including Orion (who so dominates the night sky at this time of year that you will immediately see him first when you step outside), Canis Major (containing bright Sirius, following just behind Orion in the sky), Canis Minor and the Twins of Gemini (part of what is known as the "Winter Circle" of bright stars visible at this time of year), Taurus (with its "V-shaped" Hyades, above and just east of Orion's outstretched arm holding the bow), the Pleiades (part of Taurus, but well above the Hyades), and on the other side of Orion from the Pleiades and Taurus, the zodiac constellations of Gemini, Cancer, and of course Leo.

Jupiter is indicated by a large purple arrow, below the outline of the Lion.

Now, all five of the visible planets are now in a fairly close line, from the perspective of an observer on earth. Jupiter is in fact leading a glorious procession of gods, in this order: Jupiter, Mars, Saturn, Venus, Mercury, and then the sun itself. 

Mercury is presently very close just ahead of the sun (like Venus, Mercury's orbit is interior to the earth's, only even more interior than the orbit of Venus, and thus Mercury is extremely closely "tethered" to the sun in the pre-dawn or post-sunset sky), and thus Mercury is not really visible (by the time Mercury clears the eastern horizon just prior to sunrise, the sun is so close behind that its rays lighten up the sky too much for Mercury to be visible).

However, the other four planets can be seen, if you either stay up late enough or rise up early enough and know where to look.

Following Jupiter in the current lineup is the planet Mars, visible above the eastern horizon after about 1:00 am, although you may need to wait another hour for the planet to rise far enough to clear the horizon, depending on the terrain or foliage where you live. As you can see from the chart below, Mars is following in a line behind the brightest star of Virgo, the zodiac constellation who follows Leo the Lion (giving rise to myths all around the world in which a goddess is pulled in a chariot by a lion, or else rides on the back of a lion herself, or sits in a throne flanked by lions).

The brightest star in the constellation Virgo is the star Spica. The easiest way to locate Spica (in my opinion) is to follow the line formed by the "beak" of the constellation Corvus the Crow, who appears to look directly towards that bright star). From there, Mars will be seen glowing with a strong red glow in the sky (you may even be able to see Mars and Jupiter glowing through nighttime fog, mist, or thin cloud cover, even if all the stars including Spica are not visible).

Then, if you want to see the next two planets in this glorious procession, you can either stay up all night, or rise early and go out to look east in the pre-dawn sky, where you will find Saturn located to one side of the rising form of Scorpio, coming up out of the eastern horizon as the sky begins to grow lighter before sunrise. 

Rising just behind Saturn is the brilliant planet Venus, by far the brightest of our visible planets, and the planet usually referred to as either the morning star or the evening star (depending on whether Venus appears to be "ahead of" the sun from our position on earth, or "behind the sun"). 

The January 15 - 23 edition of "This Week's Sky at a Glance" over at Sky & Telescope gives you some good pointers for seeing both Saturn and Venus in the early morning pre-dawn sky.

The screen-shot below (from the excellent open-source planetarium app Stellarium) shows all five of the visible planets, stretching in a long line from Jupiter through Mercury, followed closely by the sun (about to rise in the image below, which simulates a time of about 7:10 in the morning). 

This time, in order to keep it a little less "cluttered," I have not drawn in outlines for the various constellations, but the previous two charts show the constellations in the vicinity of Jupiter and Mars, and as you can see from the "sunrise" chart with all five planets, the great band of the Milky Way galaxy is rising up across the portion of the sky containing Saturn and Venus right now. This part of the Milky Way band -- its widest, brightest, and most visible section -- is the part which goes past Scorpio and Sagittarius. Saturn and Venus are also presently between Scorpio and Sagittarius.

Again, Mercury is not quite visible due to the proximity to the sun, and the fact that the sun will make the sky to bright to see Mercury, by the time Mercury clears the eastern horizon. However, Mercury is presently located in the vicinity of Sagittarius, close to the "feather" that can be envisioned as an ornament worn on the head of the Archer constellation.

If you want to know where on their orbits these planets are, in order to be visible in a line like this from our location on earth, there are several excellent resources on the web which enable you to view the planets circling the sun on their various orbital tracks. One such resource is the SkyMarvels site of Gary M. Winters, found here, which contains a variety of tools for visualizing the present locations of each planet, including a 3D "Solar System Scope" which enables you to change the position and angle of your viewpoint relative to the solar system's orbital tracks and the planets on those tracks.

Another is the Planets Today website, which also contains visualization tools to see where the planets are on their different orbits, as well as the astrological signs through which they are passing. The site enables different perspectives on the planets. One perspective is shown below:

As you can see, this version of the chart shows the circular tracks of the planets drawn in an "equidistant" fashion, which will introduce some inaccuracies in the perspective versus a more "to scale" version, but there are trade-offs in any type of model which tries to depict our vast solar system in a format that can be shown on a screen. 

This chart does do a good job of giving the general position of each of the planets on their orbital tracks. You can see that Jupiter, Mars and Saturn (the visible planets whose paths are exterior to the orbit of earth around the sun) are at points on their orbits which are not opposite to the earth from the sun. That's why we can see them all in the beautiful line described above.

To see Saturn, for instance, we are basically looking "across the sun" right now in order to see Saturn (visible just before the earth turns far enough to bring the sun over the curve of the horizon to an observer on the surface of the earth).

If Saturn were instead on the opposite side of earth from where it is located now (near the top of the circle, for instance, at what we could call the "12 o'clock" position on this chart), then we would be able to see Saturn at midnight instead of at sunrise, but then we would not have the same fairly closely-grouped line of planets that we have now.

Thus, you can see that the lineup we are enjoying presently is a fairly special occurrence, at least in terms of the relatively close spacing of all five planets.

I hope that you will have the opportunity to see this majestic procession of the planets in person.