The Ghost of Mars

Linguae centum sunt, oraque centum Ferrea vox. – Publius Virgilius Maro

[It (rumor) has a hundred tongues, a hundred mouths, a voice of iron]

..........In mythology, Mars has many followers: Phobos and Deimos (fear and panic, and the namesakes for Mars' two "moons"), Eris (goddess of discord, now associated with a planet Plutoid), and others. We must now add one more, the powerful demigod “Rerun”. (No, I don’t mean this “Rerun”. Or this one, for that matter.) Every August for the last few years, a email reappears on the internet, advertising how great Mars is going to look at the end of August. The emails going around often include claims that “Mars will be as big in the sky as the full Moon”, that “Mars won’t be this close again until 2258”, or that “Mars is closer than it has ever been in history”. So what’s up?

……… This email began in 2003, when there actually was a particularly close approach of Mars to the Earth. You may not remember Mars hanging in the sky like the full Moon; if so, don’t worry, you’re not crazy. Mars has a noticeably elliptical orbit, as I’ve shown in the first diagram. The Earth has an orbit that very close to circular. In the second diagram, the orbits of the Earth and Mars are drawn to scale in a way which emphasizes the oval nature of Mars’ orbit. Periodically (about once every twenty months or so), The Earth and Mars line up with the Sun, as the Earth passes Mars in its orbit. At this time, Mars appears larger than it otherwise would – but not by all that much.

……….To begin with, the closest approach (which is referred to as “opposition”, because the Sun and Mars will be on opposite sides of the sky) in 2003 was indeed close … but as big as the full Moon? The full Moon certainly dominates the sky, and seems to be huge against the celestial sphere, but actually the Moon is only about half a degree across. This means that you could stretch about 360 moons from one horizon to the other. For a angle smaller than one degree, we break one degree into sixty “minutes of arc” (this is the Babylonians’ fault – they liked dividing by sixty instead of ten), so the Moon has a size of about thirty minutes of arc. How large will Mars appear? Mars at its largest will not even reach one minute of arc, so lets go one size down and divide minutes of arc into “seconds of arc”, so that the Moon is 1800 seconds of arc (written as 1800”) across. Mars at its largest had an apparent size of 25”. Now, that’s pretty big – for Mars. We’re rather far away from Mars right now, and it currently has an apparent size of 5.8”; if you are willing to get up before dawn and look for Mars, you can find it in the constellation Gemini, above the constellation of Orion. It will be strongly red and fairly bright, but still dimmer than the two brightest stars in Orion. The chart below shows the sky at about 5 AM. The next closest approach will be at the beginning of next February, and Mars will appear to be 14” across. The claim that “Mars will appear to be as big as the full Moon” came from a claim in one of the emails that “Mars would appear as large as the full Moon when seen through a telescope”. The qualifying phrase fell out of the email quickly, which is understandable in that it doesn’t make much sense. It seems strange to say “the Moon is 75 times larger than Mars” in a way that makes it seem that you should be able to wave at Martians.

……….So, in 2003, Mars did appear larger in telescopes than it normally does, and it did appear brighter in the sky. I confess that I had no compunction about using the close approach to get the word out at that time; when we had a Mars viewing at where I was working at the time (Texas A&M University – Kingsville) had more than seventy people show up, some of whom came under the mistaken impression that they would never be able to see Mars again. Now, we’re just back in the zone where Mars appears every two years or so, large enough to show a disk when seen through a telescope

……….A lot of the confusion has been caused by the fact that the emails that had been clear and precise about Mars back in 2003 didn’t reappear each year, where the confusing emails remain. Kinda makes sense, though, since the precise emails would have had “2003” on them, and people wouldn’t be sending them six years later.

………Before I go, I have another confession: I’ve never really enjoyed observing Mars. Mars is only visible about once every two years, it does not have moons visible in a small telescope, it does not have rings, even at its closest it appears smaller than Jupiter in the telescope. Furthermore, Mars has a thin atmosphere, thick enough for there to be dust storms that can cover the entire planet, and thin enough for the dust storms to last for weeks, turning the planet into nothing more than an orange dot in the telescope. Ah well, I’ll give it another try next year, and see how it goes …

How did your Perseids go?

..........Sorry that it seems like The Messier Pro has disappeared, but a family visit, two birthdays, an anniversary, and the beginning of the new school year have driven me to take a de facto vaction for the last two weeks. There are blogs that will be up soon on Ursa Minor and Draco, Bootes and Cocrona Borealis, and Hercules.

..........The viewing event that we had for the Perseids drew thirty people, although not as many Perseids as in previous years. Too a lot of people's surprise, the next night also had a large number of Perseid meteors.

http://apod.nasa.gov/apod/ap090817.html

The Perseid Meteor Shower

..........The best laid plans o' mice and men aft gang aglee, and all that, and I am behind where I wanted to be with this blog, but if there is one thing that can bring me back with enthusiasm, it is the annual Perseid meteor shower.

(please click on the image to go to the source for the above photo)

..........Allow me to now take a step backwards. One of the romantic draws of the night sky is the unchanging nature of the sky. If you could step into a time machine and step out on the same night of the year during the time of the first dynasty in Egypt, the sky would appear the same as it does tonight. (Well, someone who has observed for years might -- and I emphasize the "might" -- notice a slight difference in the position of a handful of stars, and the sky would be darker because there would be no city lights, near or far, vomiting out photons to make sure we don't miss a Burger King or a Wal-Mart, and the sky would be turning about a noticeably different point ... but I digress, and that belongs to my discussion of the Little Dipper.) As far as the stars are concerned, the sky would be the same. Certainly, if you have done something outside with your parents the sky will be the same when you do it with your children. Even with this, the most notable occurrences are those times that act against a temporary screen against the "permanence" of the unchanging heavens. Comets, shooting stars, and the predictable motions of the planets tend to draw our eyes first, something that defines the now against a backdrop with an unchanging, uncaring nature. These things are personal, even friendly.

..........Meteor showers are also excellent opportunities to get more into astronomy, or intrduce more people to astronomy. My parents were very supportive of my astronomy, but it can be hard to pull someone out into the cold night to look at a fuzzy bit in a telescope -- after I had been able to find it. The Perseids, especially, were times that other family members came out. I have a lot of memories of going outside as a group to watch the Perseids, even one time going to a state park in Alabama to watch the display.

.........The planets move according to a predictable pattern, bright comets come and go usually as they are discovered; most bright comets are -- individually -- a once in a lifetime object, at best. Meteors (shooting stars), however, come by at about a half-dozen or so a night. Even in suburbia, a patient observer in a lawn chair can expect to see a few each week.

..........Meteors, as a whole, can be divided into three parts. Some fraction of meteors are bits and pieces of rock that never got swept up by a planet, little bits of flotsam and jetsam that have been floating around for a few billion years since the beginning of the solar system. Some are pieces from our spacecraft. NASA traces the motions of thousands of objects that could represent a threat to spacecraft, and when your speed is measured in miles per second, a small nut, bolt, heat shield, etc., can be pretty scary. I lived in central Florida for four years, thirty minutes south of the Kennedy Space Center, and it took a good bit of the romance away from meteor watching realizing that a lot of the meteors I was seeing weren't primordial gypsies, but bits and pieces of junk, haunting the orbits where they had originally been sent. (Without specifically applied energy, a satellite will continue to pass over its launch point.)

..........The third class of meteor originates in comets. When people think of a comet, the bright fuzzy head and long tail are not constant parts of the comet, but creations of the Sun as the comet comes too close. The Sun's energy causes ice, the primary constituent of a comet, to burst from the surface. The bright appearance that people think of when you think "comet" is due to the cloud of reflective ice the Sun forces off of the surface. The tail is ice and gas raised from the comet trailing behind the nucleus. After the comet has begun retreating out to the farther reaches of the solar system, that stuff doesn't disappear, but stays in the same basic orbit. The first few times that the comet passes by the Sun, the ejected dust, dirt, and associated grit stays in the same basic place in the orbit; some meteor showers, like the Leonids in November (more about that then) have big peaks aligned with the period of the comet (in the case of the Leonids, this is every 33 years). The Perseid meteor shower is the result of the Comet Swift-Tuttle, and this is old enough to get a good shower every year. The shower is also young enough to get a good shower every year -- the comet hasn't already seen all its component bits and pieces burn up in our atmosphere.

..........According to most sources the Perseids produce about sixty meteors per hour, but this (like much else) presupposed that you are observing this shower from dark site out in the country. If you are, great! If you aren't, you will see less, but any sort of sky should give you at least twenty meteors an hour on the night of its peak. (There will be meteors for several nights before the peak from this shower, but the meteors will fall off strongly after the peak on the morning of August 12th.) These meteors are called Perseids because they appear to come from a point located in the constellation Perseus (reasonable enough). The reason why is that the stream of particles crosses the Earth's orbit in such a way, and at such an angle as to seem to come from a small area on the globe of the sky.

..........Below is a map of the sky as it appears soon after dark for the first part of August (for those in the latitudes of the upper United States; if you are observing from someplace like northern Europe or Florida, there will be a minor change of perspective, some constellations will be higher or lower to the north/south, but it will be eminently usable). This is an attempt to take the inverted bowl of the sky and squash it down into something that comes more easily out of the printer. To use it, turn the sheet (presumably printing it out first) until the point on the compass at the bottom of the sheet matches the direction in which you are facing. The stars just above the horizon will be the stars at the bottom of the sheet. The constellation of Perseus is just rising in the northeast.

..........Here is the night sky a little after midnight, in early August. At this time, the number of meteors should pick up a bit, and they will tend to be brighter than before, because the Earth's atmosphere will be hitting the meteors head-on.

......... Something that I have none done successfully (so far) is to keep a diary, so to speak, of the Perseids. Using a red flashlight (to avoid ruining your ability to see at night), take a print-out of the sky and sketch the paths of meteors after you see them. If you hope to be out for some time, take several: use one for 10 PM to 10:30, one for 10:30 PM to 11, and so on. One sheet for an extended period of time for the Perseids might look like the one below. This also illustrates why they are named the "Perseids"; if you keep track of the meteors that you see on the night of the eleventh, most of them should appear to radiate out from a point in the constellation Perseus (the "radiant" of the meteor shower, naturally enough).

..........Good luck, and please stop back in and let everyone else know how your Perseids went!