Hack23.Observe Shallow-Space Objects


Hack 23. Observe Shallow-Space Objects

Things that go zip, zoom, phizz, DOH-DEE-DOH, and BOOM in the night.

For a long, long time, the stars were referred to as "The Fixed Stars" because it was believed that they were perfect and unchanging. Well, I'll forgive the ancients for thinking they were perfect. Given the lack of light pollution they had and their dependence on stars for navigation, time and date calculations, and religion, they had great knowledge and appreciation for the night sky. But they made one crucial mistake: they ignored things that didn't fit their idea of how things worked. Because it is plain to see, if you look closely enough, that the stars do change. There is a fair bit of action out in the universe.

It is possible to take note of this from your backyard (or, better, from your friends' backyards in the country). The most obvious mobile objects are the Sun and Moon. You may not even think about the Sun being a mobile astronomical object but it is a fine one (always use a safe solar filter when observing the Sun) and, of course, its motion in the sky gives us the day. The Moon, also, you've no doubt seen tooling around the sky, changing phase as it does so. And, whether you've ever explicitly noticed it or not, if you're reading this book, you're most likely aware that the planets move relative to the stars in our sky. The ancients knew about all of these objects moving in the sky and had nice, if incorrect, explanations for all this motion. But there is a lot going on they didn't know about and it makes for an interesting tour of the changing sky. You'll have to look carefully and occasionally take some notes.

2.14.1. Things That Go Zip

Shooting stars have long intrigued people, and you've almost certainly wished upon one (how did that come about, anyway?). These meteors are little bits of fluffbits of sand, flakes of dustthat hit our atmosphere and burn up in a flash. Every now and then, a bit more substantial chunk enters the atmosphere and burns as bright as the full Moon. These fireballs are impressive and almost completely random. The brightest one I ever saw I didn't actually see. I had my eye at the eyepiece when I noticed that the world was lit up as in movies featuring nuclear war. I pulled my head up but missed the fireball. However, I was rewarded with a smoke trail of the burnt up chunk that drifted away over the next five minutes. This nicely illustrates that, far from being a help, a telescope (or binocular) is actually a hindrance to meteor observing.

Like any meteor, fireballs are more likely at times of meteor showers (see Table 2-6), but they can happen anytime. Also, like any meteor, your chances of seeing one is better after midnight. After midnight, the limb of the Earth you're standing on is facing our direction of travel, making it a little more likely that a meteor will enter the atmosphere above you. If you decide to observe a meteor shower, read up on when the peak should occur. Shower peaks vary. Some are very narrow (the Leonids, for example) while others stretch over days. The Perseids are a favorite as the peak features a good number of meteors, and the peak occurs in the northern hemisphere summer, so it is warm.

To observe a shower, here is what you need: eyes. A notebook (or tape recorder) is also useful as is a nice lounge chair and blankets. Simply stretch out under the sky so that you have as clear a view of as much sky as possible and start counting meteors. You'll also want to know where the radiant is. The radiant is the point in the sky from which the meteors appear to originate. The material making up the meteors is debris that has fallen off of comets. The debris now orbits the Sun in the same orbitbut trailingas the comet. If the orbit intersects Earth's orbit, we'll get a shower as a lot of material will impact the atmosphere where the two orbits meet.

If you were to extend the trail of every meteor that is part of the shower backward, you'd find that all trails intersect at the radiant. This is actually good fun. Get a good all sky chart and sketch the meteors that you observe. After you have 15 or 20, complete the trace of eachtrail backward and see where they meet. For the Perseids, you'll find that those trailsmeet in the constellation Perseus; showers are named for the constellation in which the radiant is located. Any trails that don't intersect at this point are called sporadics. These are simply the random background noise of meteors that happen every night. Note that a good rate during a meteor shower is one meteor every few minutes. What many have in mind when they think of a meteor shower is actually a meteor storm. A good example is the 2001 display of the Leonids. The Leonids, in fact, are responsible for a number of meteor storms thanks to very dense pockets of cometary debris that impact Earth on a roughly 33-year cycle.

Table 2-6 lists the major meteor showers that occur regularly each year.

  • Maximum lists the range of dates during which the meteor shower reaches its maximum hourly rate.

  • Duration is the period during which the meteor rate is more than 50% of the maximum rate. Note that all showers may have very short and random bursts of intense activity during this period.

  • ZHR (Zenithal Hourly Rate) is the number of naked-eye meteors that would be observed from a dark sky (6.5 naked-eye limiting magnitude) if the radiant were at the zenith (directly over head). Sufficeto say, you will probably never see a meteor shower under such conditions. Thus, you will see fewer meteors, on average, than the ZHR. Again, note that bursts with ZHR of >1,000 are possible for a few minutes in most showers.

Table 2-6. Major meteor showers

Shower

Maximum

Duration

ZHR

Source comet

Quadrantids

35 January

1018 hours

120

C/1490 Y1 (?)

Lyrids

2025 April

12 days

20

C/1861 G1

hAquarids

25 May

37 days

60

Halley

S d Aquarids

2529 July

12 weeks

20

unknown

Perseids

1113 August

12 days

100

Swift-Tuttle

Orionids

2022 October

12 days

20

Halley

Taurids

18 November

12 days

20

Encke

Leonids

1619 November

0.51 day

20

Temple-Tuttle

Geminids

1214 December

0.51 day

120

3200 Phaethon


The Quadrantids are named for the now defunct constellation Quadrans Muralis (which was eliminated in 1933 when the IAU officially defined the 88 modern constellations). The radiant is in northern Boötes. This is, perhaps, the most consistently good meteor shower. However, it is most active when it's most cold. Also, the peak is very narrow. You'll need to be hardy and lucky to catch a good show, but, if you do, you'll be well rewarded.

The Leonids are a very well-known shower thanks to a larger 33-year cycle. The general stream of cometary debris that makes up the Leonid shower contains several extremely dense pockets that can cause ZHRs of 10,000 30,000 that last for a significant amount of time. Most readers will remember fantastic displays of Leonids from 19982002. The streams are dense, but narrow, so what you see depends greatly on when you observe. The usual Leonid shower is average but hang on to your hats in the early 2030s.

Meteor showers are perhaps the simplest celestial event to photograph. You need a 35mm camera and a tripod on which to mount it. Attach the widest field lens you have (35mm is best, but a 50mm lens is also good). Aim the camera at the sky. You can, but don't have to, aim at the radiant. In fact, aiming 20° to 30° away from the radiant gives you a better chance at fireballs. Focus to infinity. Set the exposure to the bulb setting (B on the exposure dial). Use a cable release to open the shutter. Be sure to use film that is ISO 400 or faster, if you're using film. Limit your exposure to a few minutes and end the exposure soon after any meteor passes through the field of view. During some showers, you can record numerous meteors in a single frame. The stars will trail (due to the rotation of the Earth) and meteors will appear as streaks that run against the grain of the star trails. Figure 2-22 was taken during the 2001 Leonid shower and shows a nice fireball passing south of Orion; note the smoke trail that drifted slowly away.


2.14.2. Things That Go Zoom and Phizz

Meteors are the fastest moving celestial objects you're likely to observe (at least until the end times). The dominant source of meteors is the next fastest: comets. Comets are often called dirty snowballs and, although it isn't precisely accurate, it's a good enough name. These are bodies of ice, dust and rock that range in size from a few hundred meters wide to tens of kilometers. Comets have very eccentric orbits. All solar system objects orbit the Sun in an elliptical orbit. Most objectsEarth, for instancehave very low eccentricity. Earth's orbit departs only 0.5% from circular. Comets, on the other hand, may get as close as a few million miles from the Sun at perihelion (nearest approach to Sol) and as far as hundreds of millions of miles at aphelion (farthest departure from Sol). When the comet is approaching the Sun, the body warms, ice melts and evaporates, and trapped gases also escape. The result is a lot of material being ejected from the body of the comet to form a coma and also a tail (or tailsusually a dust tail and an ion tail).

Figure 2-22. A fireball passes south of Orion


This can be quite impressive. Every 10 or so years, on average, a comet appears that can easily be seen with the naked eye. Most people reading this remember comet Hale-Bopp (1997) and comet Hyakutake (1996). Comet Halley (1986) was a lovely comet and comet West (1976) was stunning. Of course, the ancients knew about these too and were generally really freaked out by them. ("When beggars die there are no comets seen; the heavens themselves blaze forth the death of princes." William Shakespeare)

If they only come around every 10 years or so, you have no need to get too excited, right? Wrong. Those are just the bright, obvious comets. Every year a number of comets become visible in small telescopes or in a binocular.

You can find a good deal of information about comets from astronomical web sites. Certainly, the big ones get lots of press, but you'll want to follow it from when it's just a little speck of coma to when the party is over and the comet is on its way out to the cold outskirts of the solar system.

Also, comets present an excellent sketching opportunity [Hack #24]. Photographs have trouble showing faint things alongside bright things. That is, photographic film and electronic imaging chips have poor range. To register the faint parts of an object, you must overexpose the bright parts. Comets definitely suffer in this regard. There is a wealth of detail around the nucleus (the bright core of the coma) of the comet that is best seen at medium magnification. This almost certainly means excluding the tail (if one is visible at all). Howeverand this is the cool bitthat wealth of detail changes nightly, sometimes hourly. As jets of gas and steam explode off the surface of the comet, the appearance of the coma, inner coma, and nucleus will change. If you're patient and keep tabs on the comet, you'll get to see a dynamic celestial body being stressed mightily as it falls toward the Sun. (Of course, sometimes a comet is too far away or just too dim to show much of this detail, but you'll never know if you don't look.) Figure 2-23 shows part of my log page for 15 November 1985, including a sketch of comet Halley.

Figure 2-23. Part of my log page with a sketch of comet Halley


2.14.3. Things That Go DOH-DEE-DOH

Unlike meteors and comets, which zip and zoom around Earth's neighborhood, the minor planetsalso called asteroidsfollow more leisurely orbits farther out from the Sun (with some notable, and frightening, exceptions). For purposes of this hack, I'm going to lump in Uranus, Neptune, and Pluto with the asteroids. This will probably make all manner of astronomers angry, but, to me, there is little observational difference between Uranus, Neptune, Pluto, and the minor planets. Of the entire group, only Uranus and Neptune show a visible disk and that is only 2 or 3 arcseconds that shows no detail at all in amateur telescopes.

So, why look at them? Well, it's undeniably cool to be able to say you've seen all the planets. And, in fact, Pluto is quite the challenging object, and it's getting more challenging every day as it moves away from perihelion in 1989 and, thus, gets dimmer and dimmer (until it reaches aphelion in 2113don't wait up). So, it is a good way to hone your skills in locating and identifying objects.

You can find ephemerides for most minor planets and the three outer planets online (http://www.skyandtelescope.com is a good site for such things also see the information at the end of this hack). You'll need celestial coordinates and a good chart for all of these objects (most planetarium software will also chart the brighter minor planets and the three outer planets, as well [Hack #64]). There are excellent tips on how to put an object in your eyepiece elsewhere in this book. For now, let's assume you've put the target dead center in your eyepiece. What will you see?

Well, not much. If your target was Uranus or Neptune and you have a low magnification eyepiece in, you'll see what looks like a star. Only at relatively high magnification will the planetary disk of either planet be noticeable. If your target was Pluto or any minor planet, you'll again see what looks like a star. How do you confirm that you have the object you think you have?

Incidentally, Uranus is, under a dark sky, a naked-eye object and was observed by many early astronomers (Galileo included) before it was recognized as a planet. In fact, there are numerous recorded sightings of Uranus prior to William Herschel recognizing it as a wanderer. (The ancients called planets "wanderers" because planetsand the other solar system objects they couldn't seechange positions relative to the background stars.) Similarly, one minor planet, 4 Vesta, occasionally becomes bright enough to see without optical aid. 4 Vesta can become as bright as magnitude 5.4 and Uranus is never dimmer than magnitude 6.0. You won't mistake either of them for Venus, nor will you see them from downtown Manhattan, but from a reasonably dark sky you have a good chance at seeing them with the naked eye.


Despite looking like a star, your solar system object is going to move. Depending on the object, it will move faster or slower. Or, rather, slower or really, really slow. Generally speaking, the farther away the object is, the slower it will move against the stars (hand it to the ancients, they nailed this). Thus, Pluto crawls along. Uranus and Neptune aren't going to be confused for comets, either. What you'll have to do is sketch the field of view.

Be sure to sketch all of the stars visible and carefully note the time, date, and conditions. Either follow the object for a few hours and sketch again or compare the new view to your sketch. Has the object moved? If so, that is your target. If not, come back another night(13 days) and sketch the field again. Has one of the "stars" moved? If so, that is the one. If not, you've missed your target. You can also do this exercise with comets. In fact, it's a nice comparison to chart cometary movement with minor planet movement. You should notice a great difference. Cometary motion is sometimes noticeable in just a few minutes. Figure 2-24 shows my 19 July 1987 sketches of the asteroid 6 Hebe (pronounced "hee-bee"). On the left sketch, done just after midnight, I put x-marks next to the objects I suspected of being the asteroid. On the right sketch, done latethat evening, the asteroid has revealed itself by its movement, and is marked with an x.

Figure 2-24. Two sketches of 6 Hebe done about 23 hours apart, showing its motion


The Association of Lunar and Planetary Observers or ALPO (http://www.lpl.arizona.edu/alpo/) is an outstanding starting point for information on anything having to do with the solar system. So, for minor planets, comets, and meteors this is a good place to start.


Dr. Paul B. Jones