Libra (Reprsie)

.....Libra is the first of the constellations along the Zodiac that I have written about here. What is the Zodiac? The ecliptic is the path that the Sun takes through the sky, and the planets all stay within 7° of this line (If we skip Mercury, all the planets are within half this distance of the ecliptic). The ecliptic passes through twelve constellations (traditionally; as the constellations were given boundaries by the International Astronomical Union in 1930 the ecliptic passes through thirteen constellations), and these constellations are the Zodiac. Besides being the only zodiacal constellation with no bright deep sky objects, Libra is also the only zodiacal constellation named after an inanimate object.

.....Our constellations can be traced back to the forty-eight the Greeks had (there are eighty-eight constellations now), but the constellations of the Zodiac are much older. This makes sense because while many of the constellations can be thought of as “sky-decorations” the Zodiac allowed the ancients to track the seasons. Of the zodiacal constellations, Libra might one of the youngest.

.....Libra as the balance represents that the Sun would be in this sign at the Autumnal Equinox, when the day and night would be equal lengths, or at least it was. According to the boundaries as they are now, the Sun would be in Libra at the equinox from about 2200 BC until about 700 BC. Close to this time, if Libra did not exist as a constellation of its own, then it could have been created, to allow for twelve (one per month) constellations around the zodiac. I imagine (spoken in some Indo-Aryan root), "Okay, we need a new constellation for this point where the day and night are the same length. Remember, everything else on this list is a living thing, hence the 'zoo' in 'zodiac'." Six hours of debate follows, in which Chester (or the Indo-Aryan root for "Chester") rejects a bunch of things because they're "stupid, ten more things are dropped because Bert is allergic to shellfish, the "Claws of the Scorpion" is out because Bob of the High-Pitched Voice sat on one once, and then, "&#@% it, we'll call it the "scales", happy hour is almost over. (The bartender gave the lizard an extra squeeze, or something. Heck, I don't know what ancient beer was like.) Geez, this could have been the first "designed by committee" in history.

.....Even when not using the strict modern boundaries, the wobble of the Earth around its axis (which I discussed in a post on Ursa Minor) would have carried the equinox into Virgo, where it is now (and where it will be until AD 2450). This seems to indicate that Libra as a scales must have existed when the equinox was well inside Libra, but there are many references to these stars as Chelae, the claws of the scorpion. First, as seen below, the stars of Libra work well as the claws of the scorpion, whereas with Libra as a scales, the scorpion’s claws seem a little pathetic. The (seriously cool) names of the stars could refer to this, with Zubeleschamale translating as “the northern claw” and Zubelelgenubi as “the southern claw”. This is contested in some sources (alright, I admit it – in an unsourced article on Wikipedia) in that the Arabic (zubānā) and Akkadian (zibanitu) words for “scorpion” and “scale” are the same. Life becomes more complicated with the constellation having either four claws or four scales, but Richard Allen’s Star Names: Their Lore and Meaning considers Zubelalgubi as a degenerate version of the name “Zubelelgenubi”, and Zubenelhakrabi (the scorpion’s claw) as belonging to g Scorpii, which was apparently due to a need to invent two new claws when the other ones became Libra.

.....Why were the scorpion’s claws made into the balance of Libra (assuming, of course, that this is what happened)? Sure, there would be pressure to have twelve constellations on the Zodiac, one per month, and the reason of having a balance at the point where day and night are balanced make sense, but I have another idea why an inanimate object was added to the Zodiac where it was …

Five minutes before the first sexual harassment lawsuit

.....Zubenelgenubi is a fairly easy double star to split, with the companion star being dimmer, but not tremendously dimmer (magnitudes of 2.8 and 5.2), and about 4 minutes of arc apart, or about one-eighth the size of the Moon in the sky. I could split this double star with a pair of binoculars, so this represents an excellent opportunity to do the same, and take the first step towards learning how to observe and split double stars.

The Summer Solstice

.....(First, I hope that this is not causing anyone to skip my post on viewing Mercury, posted yesterday.) Here's what set me off.  I was directed to a story on the solstice.  First I'll give you a post on why this is the summer solstice for the northern hemisphere, and try and keep from going off on the problems I saw in the "news" story.  If you are at a cookout, the closer you are to the fire, the warmer you are.  If you are getting an English muffin out of the toaster, the closer you are to the toaster, the warmer your hand is.  In that case, it seems intuitive that the Earth is warmer when we are closer to the Sun.  It seems intuitive, but it is completely, totally, and utterly wrong.

.....The Sun does not move around the Earth (something that is only moderately difficult to get students to accept), but it looks like it does, in the same way that the sky can be treated as a big imaginary sphere.  As the Earth rotates, the North Celestial Pole (NCP) is the point in the sky that appears directly above the Earth's north pole, and the South Celestial Pole and Celestial Equator are ... I hope that you get the idea.  Since the NCP is directly overhead at the north pole, or 90° above the horizon, and the NCP is on the horizon, or at an angle of 0° above the horizon, we can see that the angular height of the NCP is the same as our latitude.  We can then track the apparent path of the Sun across the sky as the ecliptic.  Why we have seasons comes from the fact (a fact that does not depend on whether USA Today says that it is true or not) that the Earth does not rotate with its equator in the same plane in which it is orbiting, but is tilted by 23.5°.  This means that the ecliptic is tilted by 23.5° to the celestial equator (the Earth's equator projected into space) as shown.  The Earth keeps this tilt constant as it orbits, the star Polaris always above the Earth's North Pole, as shown below.  (Not to scale.)

.....The effect of this is that during part of the year, the northern hemisphere is "leaning into" the Sun, and during part of the year, the northern hemisphere is "leaning away" from the Sun.  Imagine that we pick a point on the Earth, and then trace the path that it takes over one day, a circle inscribed across the Earth at your latitude.  The effect that this has on the day comes from two things.  First, notice that the point, at the northern hemisphere summer solstice, spend the majority of its time on the sunlit side of the Earth.  It is hotter in the summer partly because the Sun is above the horizon for a longer period of time.  The northern hemisphere has its longest time of daylight on the summer solstice because the tilt of the Earth points the northern hemisphere most directly towards the Sun on this day. 

 .....The second reason why summer is hotter has to do with the height of the Sun in the sky.  In summer, a quick glance at the diagrams above should show that the Sun is much higher in the sky in the summer, as opposed to the winter.  The Sun provides the same amount of energy to the Earth at all times, but if the Sun is low in the sky that same energy is spread over a greater area, having a lesser effect.  To take Winona, Minnesota, (with a latitude of 44°) as an example, the North Star is 44° above the northern horizon, which means that the celestial equator is (90°-44°=) 46° above the southern horizon.  On the summer solstice, the Sun is  above the celestial equator, or a maximum value of 69.5° above the horizon, as opposed to the winter solstice, when the Sun is only 22.5° above the horizon, at best.  These two effects combine to make summer hotter, and, as the observant may have noticed, at all times (not just on the summer solstice), the seasons in the northern hemisphere are the exact opposite as seasons in the southern hemisphere.

.....The Earth is actually closest to the Sun in January.  This might seem strange, but really, the Earth's orbit is so close to being a perfect circle that this difference is not detectable.  One might expect the Southern Hemisphere's summer to be slightly hotter than the Northern Hemisphere's Summer, but the Southern Hemisphere is mostly water, and water has a strong moderating influence on temperature changes.

High Mercury

.....I'm not referring to the temperatures getting hotter as we approach the Summer Solstice, but the planet Mercury, which is becoming visible in the west after sunset, although I might be overstating that just a bit. Of the visible planets, Jupiter and Saturn are easy to find. Saturn is still bright in the southern sky as it gets dark, and Jupiter is bright in the sky before dawn (slowly moving into the evening sky; by September, Jupiter will be replacing Saturn in the evening). Mars is basically visible every other year, since it is only a bit farther away from the Sun as the Earth, so when the Earth passes Mars, it takes a long time to "lap" it again. Venus, like Mercury, is either a morning star or an evening star, but it is much easier to see (explanation to follow).

.....Mercury (and Venus) orbits the Sun on a smaller orbit than the Earth has. This means that Mercury (for example) can never be high in the sky at midnight because it would then have to be exactly opposite the Sun in the sky. This is clearly impossible. Taking this to a less extreme case, we can see that Mercury will always need to be close to the Sun in the sky. (Mercury also has a very elliptical orbit as well - the most elliptical orbit by far of the official planets.)

.....As you can see from the above view, Mercury is never more than 28° from the Sun in the sky at the best of times, which makes viewing Mercury very difficult. (for the nest few weeks, we are at about the middle between these two extremes.) The Sun is so bright that nothing but the Moon can be seen in the sky with the Sun, so to see a planet, we need the Sun to be below the horizon. That means that Mercury can never¹ be seen higher than 28° above the horizon, but it gets worse ...

.....The sky does not suddenly go dark when the Sun goes below the horizon because the light from the Sun is scattered in the atmosphere, so we have to wait longer, for the sky to darken, in order for Mercury to be visible. Astronomical twilight lasts until the Sun is 18° below the horizon, so in the best of times Mercury is only 10° above the horizon, while if Mercury is observed at its closest to the Sun, then Mercury will be setting at dark. Even in this best case, this means that we will be looking at Mercury through a heck of a lot more air. (We don't have to wait for the sky to be completely dark; Mercury is bright enough to show up while there still is some light in the sky, but it can be challenging to see.

.....When looking at an object 10° above the horizon, we are looking through 5.6 times as much air as when something is directly overhead. This would be like setting up your telescope at the bottom of a swimming pool - and it keeps getting worse.

.....The ecliptic is the path of the Sun across the sky. The planets have to stay near (though not on) this line, so at times when this line makes the greatest angle with the horizon (for the Northern hemisphere, this would be March), we have a better chance to see the planet. In June and early July, we are closer to the condition on the right above, than the left.


.....Until the first spacecraft went to Mercury (Mariner 10, in 1972-73), or the second (MESSENGER, in orbit as of this last March), views of Mercury were difficult to get. We had these really bad views, or wait for a solar eclipse (you guys check the footnotes, right?), so the best diagrams looked like those below, a sketch made by the astronomer Giovanni Schiaperelli. This does not look like satellite photos of Mercury (unavailable until 1973, and shown at the right). The sketch below looks more like a captcha than a planet!

Jello?

.....To see Mercury in the next few weeks look close to the horizon just after sunset. For example, tomorrow night, Mercury should appear at about 8:55 when it is about 7° above the horizon. This is less than the angle between your fingers when making the Arachnid Super Hero gesture / secret devil sign (depending on your taste in seventies metal) at arms length. You can see that if you have a wooded or hilly horizon, you are kind of hosed. The best case scenario will be on July second starting at about 9:05 when Mercury will be 10° above the horizon. Woo hoo!

1: Or during a solar eclipse, but that seems like an uncommon an extreme caveat to toss in.

Getting Mooned

....All photos in this post were taken by the author using a Canon PowerShot A630, sometimes through an 8" Celestron Schmidt-Cassegrain telescope.  The camera is held up to the telescope by an Orion SteadyPix Deluxe. 

.....If you have been outside in the evenings, you will have noticed that the Moon is getting bigger and brighter each night, heading to a full Moon tomorrow.  I have to confess that I've been (perhaps unfairly) biased against the Moon.  From the time that I started getting interested in astronomy, I have been searching in the sky for new objects that I haven't seen before, trying to find the faintest thing that I could find.  The Moon was therefore my enemy.


.....Furthermore, I could never find the "Man in the Moon".  Do you see it below?  I don't.

.....I do, however, see the rabbit.  The idea of the rabbit in the moon appears in a number of Asian legends, but I think that I'm remembering a story where the rabbit is thrown there.  Watership Down? Anyway, if you don't see it, here it is:

.....Zooming in:

.....See?  The rabbit even has a carrot.  This hatred of the Moon eased when my wife got me an attachment for my telescope allowing the camera to stay still long enough to take images through the telescope.  Here is a image of the waxing crescent Moon.

.....Photography actually is helping me enjoy observing the Moon.  One of my problems has been that it has been hard for me to adjust to most of the available maps of the Moon.  Why this is, I'm not sure.  Telescopes will invert the images as we see them. (In most cases, who cares, as there is no "up" in space - except on the Moon.)  Antonin Rükl's Atlas of the Moon was much easier to use for me (it seems that there is a revised edition - I wonder what changed), so I was able to map images to place names.  This means thta I can learn more about specific places, and then go back later and find them again!  Let's start with some of the Maria (singular: Mare), dark areas on the Moon produced from three to three and a half billion years ago when massive impactors hit the Moon hard enough to break through the surface and cause some of the still-molten interior of the Moon to boil up and re-surface the area.  Maria typically have few craters, since they were formed after most big things that were going to hit other big things already had.  (It is not a thing that could be done twice, after all.)  These features are named as if they were bodies of water because they appeared flat and featureless to early telescopic observers.

.....Let's start by looking at some of the Maria best visible during the first half of the month, and as the blog goes on, I will return to the Moon when I can to look at more features, and learn more about the ones we can identify.  The photo below has labeled areas including Mare Humboldtianum (Humboldt's Sea, named after an explorer because it lies on the border of the part of the Moon that we can see, and the part of the Moon that we can't see, always on the edge of the unknown), Mare Crisium (The Sea of Crisis), Mare Frigoris (The Sea of Cold), Lacus Somniorum (The Lake of Dreams), Mare Tranquitalis (The Sea of Tranquility), Mare Fecunditatis (the Sea of Fertility), and Mare Serenitatis (the Sea of Serenity)

.....For instance, that footprint, Mare Undorum (the Sea of Waves) and Mare Spumans (the Foaming Sea) - what's up with that?

The Arietid Meteor Shower

.....Rather unfortunately, tomorrow morning the Arietid meteor shower will be at its peak. Wow! I'm only one sentence in and I already have to explain at least four things ...

.....Meteors, or shooting stars, are small (hopefully!) particles that hit the Earth's atmosphere. Since the Earth is moving through space at about thirty kilometers every second, and the particle is going to be moving bout the same speed, the friction of the air will heat the object to glowing hot as it dissolves. Those flashing lights can come from several sources. Some of them are the flotsam and jetsam of the solar system, little pieces of leftover rock that have been floating around from the beginning of the solar system. Some have actually been blasted off of other planetary bodies, like the Moon, Mars, or asteroids - even asteroids that have been shattered in impacts in the distant past of the solar system. This is compelling stuff, but it is unfortunately not what we are talking about here. Some of these meteors can be seen each night, usually on the order of a half-dozen or so, if you're lucky. (Source)

....Meteors can also be heat tiles, nuts, bolts, tool bags, etc. After fifty years of space travel, a rather astounding amount of space junk has built up in the space around the Earth, to the point where this junk is a positive threat to space travel and new satellites that has to constantly be taken into account. (I had lived for four years in central Florida when I had cause to remember that any object launched into orbit will adopt an orbit that carried it over its original launch point periodically. How disappointing it was to have four years of meteor watching cheapened by realizing that some large fraction of what I had seen was "just" junk!)

.....Meteors can also come from comets. Consider a bright comet, passing through the inner solar system:

.....We see a comet by looking at the bright tail. We see the bright tail because the tail is made of many particles of ice and ice-covered dust released from the comet during eruptions caused by heating from the Sun. These eruptions also release a lot of dust and rocky particles from the comet, and those bits and pieces don't simply disappear when the comet (now the comet et al), moves far enough away from the Sun for the nucleus to quiet down. (We're not allowed to have something simply appear or disappear without a way to explain where it came from or where it went.) The bits that are blasted off stay in the same orbit, although they do spread out through the orbit over time. these ex-comet parts are what give rise to meteor showers.

.....Comets rarely (dramatic understatement) impact the Earth. Even when their orbits cross the Earth's orbit, the chances of the Earth and the comet both being there at the same time are remarkably tiny. However, if the comet has passed by the Sun enough that there is a great deal of debris spread throughout the orbit, the Earth can hit this. Each of these small particles becomes a meteor, and since the meteors are all coming from the same stream, and thus approaching the Earth along the comet's orbit, all these meteors appear to be coming from the same spot in the sky. (The spot where the comet's incoming orbit hits an imaginary shell representing the sky around the Earth.) This spot in the sky is called the radiant of the meteor shower, the point that meteors in this shower appear to radiate away from, and the point for this shower is in the constellation Aries. Hence, this meteor shower is refered to as the Arietid Meteor Shower. (The map is good for 4 AM tomorrow morning.)

.....I said "unfortunately" at the beginning of this post because the above star map, showing the location of the Arietid radiant, shows the sky shortly before dawn. This means that the Arietid shower is a daytime shower, and there are darn few meteors bright enough to be seen during the day. Sure, there is a chance to see meteors in the couple of hours before dawn ...

..... The typical number of meteors per hour for the peak of the Arietid meteor shower is about fifty per hour, but that is misleading. You might (on average) see fifty meteors an hour if your sky was completely without light pollution, and the radiant was directly overhead, but we aren't going to get that. A perhaps sizeable fraction of you paused at my words "before dawn" earlier. Yep, when I suggest that something cool is going to be visible in the sky, nothing kills the joy faster than the words "before dawn". At this point, I might even have negative readers for the rest of this paragraph ...

....So if you aren't willing to get up before dawn (statistically speaking, you aren't) is there any way to detect meteors during the day? As it turns out ...

.....Meteors heat up, glow, and (mostly) dissolve in the upper atmosphere. Along their paths, they leave ionized gases, knocking electrons briefly free, and this can reflect radio waves moving through the atmosphere, causing you to briefly hear radio stations more than a thousand miles away. If tomorrow morning the radio briefly cuts out, or if some station you don't recognize surges for a few seconds, that might be due to the path of a meteor.

.....Or, if you want to try to see some of these, look to the east before dawn. One advantage of these types of meteor showers is that the meteors will move comparatively slowly. This is an advantage if you, like me, tend to be able to summon fast-moving meteors simply by looking down.