Sky Roundup: November, 2007
A Comet With No Tail Leads Planets on Parade
It’s not enough that seven planets are visible in our skies for this month of November, but a once-lost comet has brightened to become a leading light, rivaling the brightness of a planet itself! Comet Holmes burst into the news in late October to grab the attention of astronomers around the world. Named for British amateur astronomer Edwin Holmes, who discovered it in 1892 while viewing the Andromeda Galaxy at the time, the faint, periodic comet was first noted then for its intermittent bursts of brightness.
The comet faded into obscurity after its last known observation in 1906 but was recovered in 1964. Since then, and still a faint comet, it showed no outward sign of its previous behavior for sudden brightening. However, by October 24, Comet Holmes brightened spectacularly in less than a day, becoming nearly a million times brighter than its former faint self.
Comet Holmes is well placed for Clevelanders to get a glimpse as it has remained fairly bright through the start of November. On a clear night, the comet shines as bright as Mars at its own minimum brightness as seen from Earth. Look for the comet in the constellation Perseus (see star chart), high in the northeast sky from early to mid-November. The comet may remain visible for a few weeks as it departs our vicinity and fades from view, though its recent brightening remains inexplicable and its future brightness unpredictable.
Just as curious as the comet’s sudden brightness is its apparent lack of a tail. Typically, as a comet orbits the Sun it will show a tail (of dust or of gas or of both) pointing away from the direction of the Sun. With optical aid, Comet Holmes appears neatly round, resembling a cotton ball, and glows somewhat bright, like a distant street lamp, but has no immediate visual proof of a tail. Even through a good binocular or a modest telescope no distinct tail is evident. Perhaps the tail faces away from us along our line of sight, which prevents us from seeing it clearly. Nevertheless, enjoy Comet Holmes as an unexpected celestial treat while it lasts.
Not only a comet to view this month, but plenty of planets are visible as well.
Since there are now only eight planets and we live on one of those eight, this is the maximum number of known planets to be viewed in the night sky at any possible time. (Recall in 2006 that poor ol’ Pluto got downsized to the status of “dwarf planet” by the International Astronomical Union.) Of course, you will need a telescope to see all of the planets best, but five can be spotted with the unaided eye. Mercury, Venus, Mars, Jupiter, and Saturn are five of the planets seen without optical aid, and have been known since antiquity. They are known as the visible planets.
To help you know that you are indeed viewing a planet, look for at least two telltale traits, namely, planets do not always appear to twinkle like stars (as long as planets are well above the local horizon), and, several visible planets will shine brighter than most stars in the night sky. This latter point is important, as it can help one to distinguish readily between star and planet.
Astronomers use the magnitude scale to determine the brightness of an object as it appears in the sky. An old classification system that dates from the time of the Greek astronomer Hipparchus, its more modern version groups celestial objects by apparent brightness and is useful for applying a relative brightness, based on a logarithmic progression, to a planet or a star.
Simply put, the magnitude scale assigns a comparative brightness to celestial objects placed on it, using positive and negative values as magnitudes. But the arrangement of the scale is counterintuitive: bright objects are assigned a low-magnitude value; dim objects, a high-magnitude value. So, the greater the value of the magnitude (or more positive), the dimmer the celestial object; conversely, the lesser the object’s magnitude value (or more negative), the brighter the object.
Hence, an object of first magnitude is brighter than an object of second magnitude, which is brighter than an object of third magnitude, and so on. Objects at sixth magnitude are at the limit of human seeing on a clear, moonless night in rural areas. Also, as the magnitude scale is logarithmic, each whole-number step in magnitude is a change of 2.51 times in brightness. So, an object of first magnitude is about 2.5 times brighter than an object of second magnitude, while a first-magnitude object is about six times brighter than a third-magnitude object.
Objects that are less than zero magnitude (or, that are negative in value) are indeed brighter than most stars, and several of the visible planets at their maximum brightness achieve magnitudes at or less than zero.
The two planets closest to the Sun, Mercury and Venus, are both seen before the Sun rises. In the pre-dawn twilight early this month, Mercury at just under zero magnitude can be tricky to spot as it hugs the local horizon, rising near to the star Spica in the constellation Virgo. Well above Mercury you can spot brilliant Venus easily if you venture outside about an hour before sunrise. It stands in conjunction with the waning crescent Moon (See Sky Roundup archive, for June 2007, for a discussion of conjunctions) halfway up in the morning sky early in the month and appears exceptionally bright at a magnitude greater than -4, outshining all of the planets.
Mars is higher in the sky as it rises well before midnight this month. Look for its bright ruddy point of light, at a magnitude of just over -1, in the constellation Gemini. Mars also starts its retrograde motion (i.e., apparent backward slippage against the background of stars) as Earth overtakes Mars in our orbit around the Sun. Mars will appear to drift slowly back (or westward) toward the constellation Taurus (the Bull), specifically toward the star Beta Tauri (which forms the north horn of the Bull’s head) by late January, 2008.
Saturn joins the parade of planets in the morning sky as it rises after midnight. It will be well placed for viewing high in the southeast as, by mid-month, it transits at sunrise. (See Sky Roundup archive for October, 2007, for a discussion of transits.) The brightly ringed planet shines at nearly first magnitude, which means that it’s a fairly bright object to see. Saturn stands in the constellation Leo (the Lion), somewhat near the bright star Regulus, the heart of the lion.
You can still catch Jupiter, too, but it’s low in the west-southwest an hour (or less by late in the month) after sunset. Still shining at near -2 magnitude (which makes it close to eight times brighter than Saturn appears), it’s a treat even in a modest telescope. Looking at it, though, so close to the local horizon may show a boiling, shimmering image as one views the great planet at a low angle through our atmosphere and its rising heat. Jupiter stands in the constellation of Ophiuchus (the Serpent Handler), a summertime constellation that is nearly invisible now as we move through mid-Autumn.
What makes this month stand out for viewing the visible planets is that they will appear at neatly spaced intervals as we see them from our vantage on Earth. For example, by mid-month and an hour before sunrise, the planets Mercury, Venus, and Saturn are spaced across the sky at intervals of nearly 28° whereas Mercury, Saturn, and Mars are spaced apart at 55° intervals. It’ll be quite a sight to see and a rare treat to view, as long as it’s clear weather.
For those two outer planets needing a telescope, namely, Uranus and Neptune, you can get your best glimpse by visiting the Mueller Observatory at the Cleveland Museum of Natural History. On Wednesday evenings (during the usual school year), when it’s clear weather outside, the Observatory is open to the public with your paid admission to the Museum. Through our vintage Warner & Swasey telescope, you can peer across the gulf of interplanetary space and see these two distant, mysterious worlds for yourself. Uranus, in the star pattern Aquarius, appears a pale greenish-gray, and Neptune, in neighboring constellation Capricornus, a pale blue.
And, if you miss Pluto as a full-fledged member of the Solar System, the renamed “dwarf planet” also appears in November skies, but is just barely visible north of Jupiter and will likely be very difficult to spot without excellent seeing, detailed star charts, and a large telescope.
So, get outside, look for the comet, and see the planets on parade!
Sky Roundup Almanac for Cleveland, November, 2007
November 1: Double shadow transit on Jupiter; Moon at Last Quarter: Moon less than 1° north of Beehive Cluster (M44)
November 3: Bright star Regulus just north of waning crescent Moon; no occultation visible
November 4: Daylight Saving Time ends; double shadow transit on Jupiter
November 7: Venus 3° north of waning crescent Moon; Taurid meteors at peak
November 6: Double shadow transit on Jupiter
November 8: Double shadow transit on Jupiter; Mercury at greatest elongation (or 19° west of the Sun) and visible in pre-dawn twilight
November 9: New Moon
November 11: Double shadow transit on Jupiter: bright star Antares just north of waxing crescent Moon; no occultation visible in Cleveland
November 12: Double shadow transit on Jupiter
November 13: Double shadow transit on Jupiter
November 15: Double shadow transit on Jupiter
November 17: First Quarter Moon; Neptune 1° north of Moon
November 19: Double shadow transit on Jupiter
November 22: Double shadow transit on Jupiter
November 24: Full Moon (for the Thanksgiving holiday weekend); Moon less than 1° from Pleiades, occulting several of its stars toward dawn
November 26: Double shadow transit on Jupiter
November 29: Double shadow transit on Jupiter
November 30: Bright star Regulus just north of waning gibbous Moon; no occultation visible
Questions or comments?
Please contact: Roy Kaelin at (216) 231-4600, x3405, or rkaelin@cmnh.org