Thursday, September 18, 2008

Hercules Revisited

Two weeks ago, we investigated the heart of Hercules, an asterism known as the Keystone. This week, let’s go farther afield in constellation Hercules and locate another asterism (recognizable star pattern) or two, a few more stars, and another globular cluster (dense ball of gravitationally bound stars).

1) Wait at least one hour after sunset to begin observing, so that twilight’s finished and your sky’s good and dark. The stars in Hercules are not terribly bright, so you’ll have the most success at a dark site, away from city lights.

2) Check the time of moonrise before you observe. The waning (shrinking) Moon will rise a bit later than the night before, each night this week. Because Hercules is a relatively dim constellation, you won’t want moonlight to interfere with your stellar scavenger hunt.

3) Locate the Keystone. If you need assistance to do so, review my earlier post.

4) If you “connect the dots” to a naked-eye star above each of the top (northern) two Keystone stars and a naked-eye star below each of the bottom (southern) two Keystone stars, you'll see the asterism known as the Bowtie. The Keystone becomes the center knot in the Bowtie.

5) The southernmost star of the Bowtie is Kornephoros (core-NAY-for-uss), which is Greek for club bearer, a reference to Hercules’s weapon of choice. Kornephoros is a yellow giant and the brightest star in the constellation.

6) Immediately southwest of Kornephoros are three somewhat dim stars that form a small triangle. This triangle is the asterism known as the Club, and you’ll need dark skies to spot it. The triangle star closest to Kornephoros has no traditional name, so we call it Gamma for its star catalog designation. Continuing in a line from Kornephoros through Gamma, we come to Marsic (MAHR-sick), Arabic for elbow. The third triangle star, to the east, is Cujam (KOO-zhahm), from the Latin for club.

7) Southeast of Kornephoros is a slightly dimmer star called Rasalgethi (rah-sahl-GAYTH-ee). The name of this red supergiant star is from the Arabic for the kneeler’s head. You may recall from my previous post that an early Greek name for this constellation was “Kneeling One.”

Don’t confuse Rasalgethi with Rasalhague (RAH-sahl-hayg), the brightest star in the constellation Ophiuchus the Serpent Handler. Rasalhague is brighter than Rasalgethi and a little farther east.

8) Let’s finish up our survey of the strongman with a deep-sky object. Although M13 is the best known globular cluster in Hercules, he harbors a second glob from the Messier catalog: M92. M92 is perhaps not quite as celebrated as M13, but it’s a very pretty object nonetheless.

You’ll need a telescope for a satisfying view of M92. If you make an imaginary equilateral triangle using the top two stars of the Keystone as the base, and then point your telescope slightly east of the top of the triangle, you’ll run into M92.

If you compare M13 to M92 in your telescope, you’ll note that M92 is a bit smaller and dimmer than M13. However, M92 has an asymmetrical appearance that I find irresistible in a glob. Globular clusters are, after all, the renegades of the Milky Way, arranged in a spherical “halo” around the core of the galaxy rather than huddled in the platter-shaped disk, where all the other stellar material congregates.

Side view of Milky Way, showing halo of globular clusters around galactic core
Diagram by
Richard Powell

Charming imperfection and a wayward nature: a killer combo. As galactic sugar piles go, M92 can always be relied upon to satisfy my sweet tooth.

Thursday, September 11, 2008

Shine On, Harvest Moon

The Harvest Moon is the Full Moon that occurs nearest the autumnal equinox, also known as the fall equinox. The Harvest Moon usually occurs in September and occasionally in October. This year, the Harvest Moon falls on Monday, September 15, one week prior to the fall equinox on Monday, September 22.

The fall equinox marks our official first day of autumn. Astronomically speaking, the equinox (EE-kwih-nocks) is the moment in time, calculated to the minute, when the center of the Sun is above the Earth’s equator. This occurs only twice a year, in the spring and in the fall.

To understand why equinoxes occur only twice a year, you need to remember that Earth’s axis is tilted 23.5 degrees with respect to its orbit around the Sun. If it weren't, then Earth’s equator would line up with the plane of Earth’s orbit around the Sun, and the center of the Sun would always be above the equator. Of course, we would have no seasons. The temperatures at a given location on the globe would not vary much over the course of a year. We wouldn’t see the Sun arcing high in the sky in the summer or arcing low--nearer the southern horizon--in the winter. It would pretty much follow the same course in the sky, sunrise to sunset, day after day, all year long.

Earth’s tilt with respect to its orbit around the Sun
Image courtesy of Tau’olunga

But that's simply not the case, is it? Our 23.5-degrees-off-kilter planet ensures that here in the Northern Hemisphere we tilt toward the Sun in the summer and experience the heating effect of direct sunlight, and then tilt away from the Sun in the winter and get the chilling effect of indirect sunlight. In spring and fall, midway between the two temperature range (and tilt) extremes, we enjoy moderate temperatures.

Another way to think of the equinox is that it’s when the ecliptic (the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth) intersects with the celestial equator (the imaginary line that represents the plane of the Earth’s equator, extended out into space). The diagram below shows the twice-yearly intersection of these two lines at the equinoxes.

A Tale of Two Planes: Ecliptic and Celestial Equator
Diagram by
Dr. Guy Worthey

The Earth’s sunward tilt in the summer puts the plane of the equator below the Sun’s center (which corresponds to the ecliptic), and the Earth’s sun-shunning tilt in the winter puts the plane of the equator above the Sun's center. Only during the in-between seasons of Earth’s orbit--spring and fall--can the Sun’s center intersect with the celestial equator and therefore be directly above the Earth’s equator.

The Harvest Moon was so named because it occurs during the traditional peak of crop harvesting in the Northern Hemisphere. In times past, farmers rushing to bring in their crops could continue working by moonlight after the sun had set.

The Harvest Moon was particularly revered by hard-working farmers of old for a reason that has astronomy at its root. Normally, the Moon rises around 50 minutes later each successive night. However, for several days around Harvest Moon, the Moon rises only 25 to 30 minutes later than the evening before. This eliminates or shortens the period of darkness between sunset and moonrise that might otherwise hamper farmers still out standing in their fields. These quicker-than-usual moonrises are so noticeable they’ve given rise to a mistaken but persistent belief that the Moon rises at the same time on successive evenings around the Harvest Moon.

The astronomical reason for the shortened time between moonrises around Harvest Moon is that the angle between the ecliptic and the horizon is very narrow in September. Contrast this with the beginning of spring--in March--when the angle is very steep. The Moon appears to follow approximately the same path across the sky as the Sun, that is, the ecliptic. As the Moon moves along the ecliptic from night to night in September, its shallow angle relative to the horizon ensures that the turning Earth will reach the Moon’s new position on the ecliptic sooner than it would if the ecliptic were steeply oriented to our horizon.

A picture is worth a thousand words, so take a look at this diagram by Patrick Moore, comparing the ecliptic’s angle relative to the horizon in both spring and fall.

The old Anglo-Saxon word for autumn, “hoerfest,” gave us our mouth-watering word “harvest.” Thoughts of the harvest naturally turn to thoughts of food and drink. The Chinese Mid-Autumn Festival, a harvest celebration which occurs near the autumnal equinox, falls this year on Sunday, September 14. The festival includes the ritual eating of mooncakes, round pastries traditionally filled with lotus seed paste and sometimes a salted egg yolk to symbolize the Full Moon. If you don’t have an Asian market offering mooncakes in your area, try this modern recipe for mooncakes.

If mooncakes aren’t your cup of tea, you can always fall back on an American classic: the MoonPie. Developed in 1917 as a snack for coal miners, it was purposely fashioned to look like the Full Moon (and therefore, substantial). The marshmallow and graham cracker treat is available with a chocolate, vanilla, or banana-flavored coating.

For a walk on the wild side, try this Harvest Moon cocktail. As for me, I’ll be toasting in the Harvest Moon with a tall glass of ice-cold apple cider.

Thursday, September 4, 2008

The Heart of Hercules

The constellation of Hercules is believed to be one of the oldest observed star groupings, as evidenced by the large number of ancient names attributed to it. Greek astronomers originally knew this star group by a name that meant Kneeling One. Its identification with the strongman of Greek legend came a bit later.

The origin of the name Hercules can be traced to the Phoenician word for traveler. Perhaps this was a reference to the famous Twelve Labors of Hercules, which kept him busy all over the ancient world. Hercules’s bags were permanently packed for parts unknown and high adventure.

Hercules in 17th century star atlas (Keystone asterism highlighted)
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Let’s find Hercules by locating the well-known asterism (recognizable star pattern) that lies at the core of the constellation.

1) Facing west (the direction in which the sun sets), locate Arcturus, the brightest star in Bootes, and Alphecca, the brightest star in Corona Borealis. If you’re just tuning in to this blog and need assistance, read my previous two posts.

Charts created with Your Sky

2) Draw an imaginary line from Arcturus to Alphecca and keep going to the next fairly bright star. This star has no traditional name, so we call it Zeta (ZAY-tuh) for its star catalog designation. Zeta is situated at the lower right corner of the Keystone, a four-star asterism forming a quadrilateral that marks the torso of Hercules. Can you see all four stars? If not, try again at a darker location. The stars of the Keystone are not terribly bright, and they may be obscured in brightly lit urban or suburban areas.

A keystone is an architectural element: the wedge-shaped piece at the top of an arch. In fact, it’s the key element of the arch, locking the other pieces into place.

3) The Keystone contains one of the most spectacular globular clusters in the Milky Way: the Great Hercules Globular Cluster, also known as M13. M13 is shorthand for Messier 13 (MESS-ee-yay); it’s the 13th object in the catalog of the famed 18th century French astronomer Charles Messier. This catalog contains some of the finest objects in the night sky and is widely used by amateur astronomers as an observing list.

A globular cluster is a dense ball of gravitationally bound stars. There are at least 150 known globular clusters in our home galaxy, the Milky Way. M13 contains hundreds of thousands of stars.

M13 was discovered in 1714 by Edmund Halley (of Halley’s Comet fame). Because telescopes of his day were not powerful enough to resolve the cluster into stars, that is, separate it into distinct points of light, Halley saw only a fuzzy blob. Since it resembled a nebula, a cloud of gas and dust, for a time the object bore the name of “Halley’s Nebula.”

In 1974, the giant radio telescope at Arecibo Observatory in Puerto Rico targeted M13 to broadcast one of the first messages from Earth intended for intelligent extra-terrestrial life forms.

If you have binoculars, train them on the spot marked on the map above and look for the fuzzy blob that Edmund Halley saw. If you have a telescope, use it to resolve the showstopper cluster into individual stars. Then perhaps you’ll see why I refer to globular clusters as “galactic sugar piles.”