Thursday, November 27, 2008

Giving Thanks

These are a few of my favorite things.

Dance of the Planets
Watching the naked-eye planets (Mercury, Venus, Mars, Jupiter, Saturn) rhythmically huddle and disperse is like watching an intricate dance. These planetary alignments are the result of our earthbound view of the naked-eye planets moving in their respective orbits around the Sun. Some are traveling faster than Earth; some are traveling slower. The resulting variety of configurations gives us a sense of the “clockwork of the universe” and reminds us that everything is in motion.

Solar system diagram created with Solar System Live

Be sure to look southwest after sunset on Monday, December 1, when Venus, Jupiter, and the crescent Moon will bunch up in the twilit sky. Venus will be the brighter of the two star-like objects you see, and Jupiter the fainter. The waxing (growing) crescent Moon will be about 15 percent illuminated.

While you're enjoying the alignment, look for earthshine on the Moon. Earthshine is sunlight reflecting off the surface of the Earth and illuminating the dark part of the Moon. Even though the bright crescent is the only part of the Moon’s face being directly illuminated by the Sun, if you look carefully, you’ll see that the rest of the Moon’s face is glowing faintly with reflected glory: earthshine.

Shooting Stars
Wishing on a “falling star” is one of the guilty pleasures of the stargazer. Even after you know that falling stars, aka "shooting stars," are meteors, you can still appreciate this charming holdover from early folkloric traditions.

A meteor is the streak of light we see in the sky when a bit of dust or space debris hits Earth’s atmosphere at high velocity. A meteor shower occurs when Earth encounters a stream of debris left behind by a comet’s close approach to the Sun. Sporadics are meteors not associated with a particular shower.

Meteor showers occur reliably around the same time every year, as Earth returns to the point in its orbit where the debris field lies. Whenever you like, you can find out what meteor showers are currently active by visiting this website and scrolling down to the Meteor Shower Calendar.

Meteor storm - 1889 engraving by Adolf Vollmy

If you’re lucky, you might someday see a meteor storm, a very intense meteor shower with a high frequency of meteors. One of my top ten stargazing experiences ever was viewing the 2001 Leonid meteor storm. The meteors came so fast and furious, it was a bit like watching a fireworks show. I was with a large group of fellow amateur astronomers at a dark-sky site, and the oohs and aahs from the crowd as particularly fine meteors blazed across the sky just added to the ambiance of the event. We all knew we were seeing something rather spectacular.

Cosmic Dust Bunnies
Interplanetary space is not empty. Our solar system contains a vast number of minute particles, some left over from the time of planet formation and some ejected from passing comets or asteroid collisions. Light from our Sun illuminates the myriad particles lying in the solar system plane, and sometimes we see the reflected sunlight as a luminous pillar of light called the zodiacal light.

The zodiacal light (zoh-DYE-uh-kull) is a large, roughly triangular or cone-shaped glow in the night sky, extending upwards from either the eastern or western horizon. The base of the triangle or cone— the widest part of the glow— is at the horizon. The zodiacal light is typically seen in the spring above the western horizon after sunset and in autumn above the eastern horizon before sunrise.

If you're fortunate enough to view the zodiacal light when transparency (atmospheric clarity) is good and the sky is inky black, you may notice the light pillar has a yellowish cast. Our Sun is, after all, a yellow star, so it shines with a golden light.

Cone-shaped glow of zodiacal light above sunset glow
Image source: ESO

The zodiacal light is subtle and can be quite challenging for the beginner to spot. A dark site away from urban and suburban night lighting is critical for success, as is avoidance of a moonlit night.

Believe me, this phenomena is worth a little extra effort. It’s quite a rush when you spot it, especially when you then start thinking about what you’re looking at— vintage dust from when the solar system formed. The goofy image that always pops into my head is the Sun as a clean freak with a big flashlight, showing us what lurks under the furniture.

The Way
My personal pet name for our home galaxy, the Milky Way, is simply “the Way,” because it represents for me a way of being and a way of seeing. I learned an important lesson a number of years ago on a camping and observing trip in southern Arizona. I had spent the better part of a night looking through the eyepiece of my telescope, hunting various galaxies, globular clusters, and nebulas on an observing list and methodically ticking off those “faint fuzzies” as I found them. Straightening up from the eyepiece to stretch my back, I faced south and audibly gasped. The woods south of the clearing where I and my partner were observing were on fire!

To be continued…

Thursday, November 20, 2008

The Reclusive Ram

Aries the Ram is a bit of a recluse, hiding in plain sight in the late autumn sky. Although not terribly impressive to behold, Aries is historically an important constellation. In the astrological zodiacs of numerous ancient civilizations, it was considered the first zodiacal constellation, or first sign. This designation may have been made when it was noted by the ancients that the Sun was positioned in Aries at the spring equinox.

The spring equinox is the moment when the center of the Sun is directly above the Earth’s equator. This occurs only twice a year; the other occurrence is known as the fall equinox. Another name sometimes given for the spring equinox is first point of Aries, which, of course, derives from old astrological traditions.

Aries in Bayer's 1603 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

The “first sign” tradition continues today; Aries is usually listed first in horoscope columns found in newspapers and magazines. You’ll also note that the conventional date range given by astrologers for people “born in the sign of Aries” begins on March 21; spring equinox typically occurs on or near March 20.

By the way, the Sun is no longer in Aries at the spring equinox. Due to precession— sometimes called precession of the equinoxes— which is the wobbling-top motion of Earth’s axis, the equinoxes occur a few minutes earlier each year. As a result, at each spring equinox, the Sun is not in the same position it was the previous year against the background stars, as seen from Earth. If we track the Sun’s position over the years at a fixed point in time (such as the spring equinox), from our perspective on Earth the Sun would appear to move through all 12 zodiacal constellations in reverse. As a result of precession, the Sun is now in Pisces, the twelfth zodiacal constellation, at the spring equinox.

Aries in Middleton's 1842 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

In mythology, Aries represents the ram whose golden fleece was famously sought by Jason and the Argonauts. Let’s conduct our own search for the storied sheep.

1) Wait about two hours after sunset to begin observing, so that Aries is high enough in the sky.

2) Face east. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your back to the west, and you’ll be facing approximately east.

Star maps created with Your Sky

3) Find the Great Square of Pegasus, which is nearly overhead, and the Pleiades star cluster, which is low in the east. These are both easy to spot using the star map above. Midway between them is Aries. Look for a small, curved line of three stars. This is the most notable asterism (recognizable star pattern) in Aries. It always remind me of a comma, so I call this asterism the Comma. Unfortunately, there’s nothing in the scattering of stars in Aries that particularly resembles a ram, unless you want to think of the Comma as a curving ram’s horn.

4) The brightest star in Aries is Hamal (hah-MAHL), which comes from the Arabic for lamb. Other ancient names for this star had meanings like ram’s eye, horn star, and leading one (the ram that led the celestial flock). Hamal is an orange giant star, 66 light years away. One light year is the distance light travels in one Earth year, nearly six trillion miles.

5) The second brightest star in Aries is Sheratan (SHER-uh-tunn), from the Arabic for the pair, a reference to Sheratan and the final star in the Comma, which we’ll look at next. Sheratan is a white star. When you compare it to Hamal, can you see that orange giant Hamal has a more golden hue?

6) Mesarthim (mess-ahr-TEEM) is from the Hebrew for servants. Mesarthim, a white star, may only be Aries’ fourth brightest but it’s known as the first star of Aries. This is because in antiquity, when the spring equinox was in Aries, Mesarthim was the closest Ram star to the equinox. Physically, the equinox can be described as the intersection of the Sun’s path with the celestial equator, the imaginary line that represents the Earth’s equator extended out into space.

7) Although the third brightest star in Aries has no traditional name and is instead saddled with the uninspired moniker Flamsteed 41, it does manage to be part of a funky little asterism. The blue-white dwarf star is the brightest member of a four-star asterism known as the Northern Fly, riding on the ram’s back. To find Flamsteed 41 and the Northern Fly, look northeast of Hamal and east of the constellation Triangulum.

In its glory days, this asterism was part of a now-defunct constellation by the same name, which was introduced in the 17th century. Now it’s merely a curiosity for naked-eye stargazers like us and, I imagine, a constant irritant for our celestial ram.

The Ram and the Fly in Bode's 1801 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Thursday, November 13, 2008

The Cockeyed Moon & the Listing Earth: Part I

Question posed by a reader: I’ve noticed that the Moon doesn’t always rise in the same spot on the horizon. Is there a pattern to this over the course of a month and over the course of a year?

Answer: Yes!

You could say that our Moon marches to the beat of a different drum. Any self-respecting natural satellite would orbit directly above its planet’s equator, ensuring no change from day to day in the position of the moonrise on the eastern horizon. Not so with our cockeyed Moon.

Instead, the Moon’s path as it orbits Earth is much more closely aligned with the ecliptic, the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth.

The Cockeyed Moon
Image source: NASA

Hopefully, you’ve noticed that the Sun rises at different spots on the eastern horizon over the course of a year. It rises incrementally farther to the north as we approach the summer solstice in June. Then it reverses course, rising incrementally farther to the south as we approach the winter solstice in December. This occurs because Earth lists a bit. That’s right, our home planet is permanently tilted with respect to its orbit around the Sun. In other words, the Sun is not above the Earth’s equator all the time. If it were, we would see no change in the position of each sunrise.

The Listing Earth
Image courtesy of Tau’olunga

Because it roughly follows the path of the ecliptic, the Moon has a rise cycle with a northern and a southern extreme, just like the Sun does. The Moon, however, goes through its cycle in one month, not one year. In any given lunar month, the moonrise travels back and forth between an extreme northeast position on the horizon and an extreme southeast position. This is observable. Check it out!

First, find a location with a clear view of the eastern horizon. You’ll need to use this same location on several dates over the course of one lunar month. A lunar month or lunar phase cycle is the time it takes the Moon to orbit the Earth and reach the phase it started from, as seen from Earth (for example, Full Moon to Full Moon or 8% waxing crescent to 8% waxing crescent). The average length of a lunar month is 29.5 days.

Second, decide how you will mark moonrise positions on the horizon. If you have a horizon with a lot of topographical and/or manmade landmarks, you could simply make notes describing where the Moon rose with reference to specific landmarks. You may want to sketch the horizon and its landmarks and mark the different moonrise positions and their dates on your sketch. You may even decide to capture the moonrises photographically. It’s up to you.

Third, use the named phases of the Moon (New Moon, First Quarter, Full Moon, Third Quarter) to streamline your data capture. The dates of these phases for any given month can be found on most calendars. If you simply record the moonrise position on the dates of five consecutive named moon phases, you’ll have what you need. A First Quarter Moon rises a couple hours before or after 12 noon, Full Moon rises around sunset, and a Third Quarter Moon rises around midnight or later. Check this website for moonrise times in your area.

Phases of the Moon
Image source: NASA StarChild

New Moon is a bit tricky. Because the Moon travels so close to the Sun in the daytime sky at New Moon, you won’t be able to observe the moonrise at New Moon. However, you can note the approximate location of sunrise on the eastern horizon at New Moon and use that for your moonrise position. It’ll be very close to the Moon’s actual position. (WARNING: Never look directly at the Sun, as this will permanently damage your eyes!)

When you complete this project, your five position markers will show you that the moonrise does indeed swing back and forth between a northeast and a southeast extreme (not necessarily in that order).

We’ll revisit this topic at a later date in Part Two, when we’ll take a look at moonrise positions and the pattern that emerges over the course of a year.