Go Stargazing! September Edition

Venus and Mars have left Saturn behind in the night sky (check out my earlier blog on the position of the planets). You can spot the star Spica in between Mars and Venus during this time of year. (Spica is similar to Mars in brightness and closer to Venus than to Mars). 

 Cloud structure in The Venusian atmosphere,
revealed by ultraviolet observations

September is the last full month to observe Venus at dusk. That’s because Venus has by now come around to Earth’s side of the sun on its faster, inner orbit.  Thus, Venus now begins to overtake the Earth, passing between the Earth and sun on October 29.  We’ll therefore see Venus shift farther to the left of Mars and then drop down below it.  In October, Venus exits the evening sky quite quickly as it shifts back towards the sun.  September and October 2010 is an excellent period for observing Venus’ crescent phase in telescopes.  Anytime Venus is on our side of the sun, more of its night side faces us, resulting in a crescent like appearance when magnified.

Saturn is far to the lower right of Venus and Mars as you face west at dusk.  You’ll need a horizon clear of tall buildings and trees to see it before it sets.  You’ll also need to look early in the month, as Saturn is practically behind the sun by month’s end.  

Jupiter dominates this month’s skies.  On Tuesday morning, September 21, Earth aligns with the sun and Jupiter, bringing Jupiter to opposition (because the sun and Jupiter are then on opposite sides of the Earth).  On the night of September 20-21 we see Jupiter rise at sundown and set at sunup—Jupiter is up literally all night long.  During the whole month, though, Jupiter is visible virtually the whole night.  It outshines all stars in the sky, so it’s easy to find.  Face east in late evening or south southwest at dawn to see it.  The planet Uranus is less than one degree above Jupiter this month; the two planets are closest on September 18.

The Big Dipper is setting in the northwest at dusk; you now need a horizon clear of trees and tall buildings to get a good look at it. You can extend the curve of its handle to ‘arc to Arcturus’, which is in the west at dusk tonight.  Arcturus, by the way, is the fourth brightest star we ever see at night, but the brightest one Americans ever see on a September evening.

As the Dipper gets lower, look for five stars in the shape of an ‘M’ directly across the North Star from the Big Dipper’s handle.  This is Cassiopeia, the Queen—the ‘M’ is the outline of her throne.  Her stars are about as bright as the North Star and the stars of the Big Dipper, so she’s not too hard to find. 

星空下的汗腾格里峰 / Mt. Khan Tengri under Galaxy
Creative Commons License photo credit: livepine

High overhead, look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair.   This triangle was up all night long from June to early August, hence its name.  Scorpius, the Scorpion, is in the southwest at dusk.  Sagittarius, the Archer, known for its ‘teapot’ asterism, is to its left.  Between these two star patterns is the center of our Milky Way—the brightest part of that band as wee see it.  On a cloudless night far from the big city, see if you notice the Milky Way glow near the ‘teapot’ of Sagittarius. 

Look for the Great Square of Pegasus rising in the east.  The vast stretch of sky under Pegasus is largely devoid of bright stars—ancients called this the ‘Celestial Sea”. 

Moon Phases in September 2010:

Last Quarter                  September 1, 12:22 am, September 30, 10:52 pm

New Moon                       September 8, 5:29 am

1st Quarter                     September 15, 12:49 am 

Full Moon                        September 23, 4:18 am

At 10:13 pm on Wednesday, September 22, the sun is directly overhead at the equator.  As a result, everyone on earth has the same amount of daylight and the same amount of night.  That’s why it is called the equinox (‘equal night’ in Latin).  In the Northern Hemisphere, we’ve seen the days get a little shorter and the midday Sun a little lower each day since June 21.  For us, the season changes from summer to fall at the equinox.  In the Southern Hemisphere, people have seen the days lengthen and the midday Sun get a little higher each day since June.  For them, the season changes from winter to spring.

This month, see a ‘Hairy Star!’

An unexpected visitor graces our skies this month.  Comet Lulin is now visible through binoculars in late evening and morning skies.  It makes its closest approach to Earth on February 24, when it may even be dimly visible to the naked eye!

Comet Hale-Bopp
Creative Commons License photo credit: tlindenbaum

Comets are made of ice and dust and are often called ‘dirty snowballs.’ They are believed to be left over from the formation of the solar system.  As comets approach the sun, ice changes into gas and the dust embedded within the ice is released.  A cloud of particles expands out to form a coma around the comet’s solid nucleus. This coma may be a hundred thousand miles across. Radiation pressure of sunlight and the powerful solar wind sweep gases and dust off of the comet, forming tails pointing away from the Sun. The coma and tails of a comet reminded the ancient Greeks of hair; the Greek word ‘kometes’ means ‘hairy.’   

Astronomers traditionally name comets after their discoverers.  On July 11, 2007, Lin Chen-Sheng of Lulin Observatory in Nantou, Taiwan took some photographs of the sky.  The photos were part of the Lulin Sky Survey, in which astronomers search the sky for Near-Earth Objects which might pose a risk of colliding with Earth.  One of his students, Ye Quanzhi, spotted what he thought was an asteroid in three of the pictures.  Closer observation, however, revealed the coma of a comet.  Officially designated C/2007 N3, the comet was named Lulin after the observatory where it was discovered. 

Here are some interesting facts about Comet Lulin’s orbit:

The eccentricityof an orbit describes its shape.  Bound orbits are ellipses with eccentricities between 0 and 1; 0 is a perfect circle while 1 is a parabola.  Lulin has an eccentricity of 0.9999948, almost 1.  This indicates an orbit so oblong that Lulin won’t return to the inner solar system for about 50 million years.  Some sources indicate an eccentricity slightly greater than 1.  In that case, Lulin will never again approach the Sun.

Lulin was closest to the Sun (at perihelion) on January 10.  But it approached the Sun from the far side (from our perspective).  Thus, as Lulin recedes from the Sun, it approaches Earth, with closest approach on February 24.  Not to worry, though–even at its closest, Lulin will be about 150 times as far away as the Moon.

Many comets’ orbits are highly inclined to ours.  (An inclination of 0 degrees would describe an orbit in the same plane as Earth’s orbit.)  Comet Lulin has an inclination of 178.37 degrees.   This inclination of almost 180 degrees puts Lulin back in the plane of the solar system, orbiting backwards compared to the planets’ orbits. 

Since Lulin orbits almost in Earth’s orbital plane, we see not only a tail but an ‘anti-tail.’  This is dust and debris left behind as the comet moves on its path.  Lulin is now moving away from the Sun, so the dust it leaves behind seems to point towards the Sun. The true tail of a comet always points away from the Sun (and therefore, the tail leads the comet as it moves away from the Sun). 

The Hale-Bopp Comet
Creative Commons License photo credit: Wolfiewolf

Because Lulin is roughly in the plane of the solar system, traveling backwards, it appears against the same zodiac band where we find the Sun, Moon, and planets in our sky.  As I type this, Lulin is among the stars of Virgo, the Virgin, moving towards Leo, the Lion. 

As we pass more or less between the Sun and Lulin next week, we’ll see it in Leo, first near Saturn and then near the bright star Regulus.  Lulin will be rising in the east at about dusk, highest in the sky about midnight, and setting in the west just before dawn.  Since Lulin and Earth are going in opposite directions, we see Lulin move quite noticeably night to night. 

This page has some finder charts for Lulin.  Some observers have reported seeing Lulin naked-eye, at the threshold of visibility.  You must get far from city lights, therefore, to see it without binoculars or a telescope.  Remember to scan the sky for a diffuse object about half as big across as the full Moon (and much dimmer than that), not a point of light.  Those who saw the spectacular comets Hyakutake and Hale-Bopp in the ’90s should keep in mind that Lulin will be barely (if at all) be visible to the unaided eye and will not come close to their displays.  If you find Lulin, see if you can follow it as it gets dimmer but higher in the evening sky in March. 

Once it fades away, we’ll never see it again. 

Happy New Year!

The New Moon of Monday, September 29, is an important one to many people of the world. In the Hebrew Calendar, it marks Rosh Hashanah (literally, ‘head of the year’) which is the beginning of year 5769. On Tuesday night, September 30, Muslims across the world will see the first slender crescent of this lunar cycle. That will mark the end of Ramadan and the beginning of the next month, Shawwal. 1 Shawwal is ‘Eid ul Fitr, one of the greatest holidays in the Islamic calendar. This week, then, is a good time to think about the Moon, why it’s here, how it orbits, and how we have used it to measure time.

Moon and stars
Creative Commons License photo credit: joiseyshowaa

Unlike our months, Hebrew and Islamic months begin with the New Moon. Because twelve lunar months add to only 354 days, less than the 365.25 day solar year, an extra month is occasionally needed to keep the months roughly aligned with the seasons. In a 19 year cycle, years 3, 6, 8, 11, 14, 17, and 19 have the extra month. The year that is ending, 5678, is number 11 in its cycle and was a leap year.

Interestingly, the Jewish year has two ‘beginnings’. Tishrei (the month which begins now) is the first month of the civil calendar, and the month where 5678 becomes 5679. However, it is actually the seventh month of the religious calendar, which begins at Nisan (the month of Passover).

The Islamic calendar functions slightly differently. Its months begin with the first visible crescent low in the west at dusk, which is not with the actual New Moon. Keep in mind that at New Moon, the Moon is in line with the Earth and Sun, and the entire near side of the Moon has nighttime (and is therefore dark). The New Moon is visible, therefore, only if it blocks the Sun during an eclipse.

Since this New Moon occurs early Monday morning, the 29th, we expect it to be visible by Tuesday evening, the 30th. Observant Muslims, then, will continue to fast in daylight hours Monday and Tuesday. Upon seeing the Moon Tuesday night, they will know that Ramadan has become Shawwal, and they may break their fast on Wednesday.

Due to early controversy as to which years would have it, Muhammad outlawed the 13th month that kept Islamic months tied to the seasons. As a result, Ramadan (and each other month in that calendar) begins 11 days earlier each year according to our Gregorian calendar.

The Moon is Earth’s only natural satellite, orbiting our planet once every 27.3 days. However, a cycle of moon phases (say, from New Moon to the next New Moon), takes 29.54 days. This is because the Earth itself is moving during each 27.3 day Moon orbit. Since it is much easier to observe the Moon’s changing phase cycle than to observe the Sun directly, the 29.54 day phase cycle was the basis of many ancient calendars. Words for ‘moon’ and ‘month’ are related in English and are identical in many other languages. There is some evidence that our word ‘moon’ is ultimately related to an Indo-European word for ‘measure.’ Given how long we’ve measured time by the Moon, it is easy to take its presence for granted.

Released to Public: Jupiter Montage (NASA)
Creative Commons License photo credit: pingnews.com

However, our Moon is quite remarkable in several ways. Moons in our solar system are generally much smaller than the planet they orbit. Jupiter and Saturn, for example, are about 25 times bigger across than their biggest moons. Earth, though, is only 3.67 times the diameter of our Moon. Also, moons usually orbit in the same plane as their planet’s equator. Our Moon, though, orbits within about 5 degrees of Earth’s orbital plane, called the ecliptic, which is not the plane of the equator since Earth is tilted 23.5 degrees on its axis.

This leads most astronomers to believe that the Moon did not form with the Earth, but is the result of a collision with with an object roughly the size of Mars. According to this theory, the impactor (sometimes called ‘Theia’) struck a glancing blow on the Earth and was completely destroyed, and the Moon formed from the debris of Theia’s and Earth’s mantles.

This impact is what left Earth with a Moon much larger than what a planet Earth’s size would normally have, and left that Moon near Earth’s orbital plane (where the impact occured). Our relatively big moon has crucial effects not only on our tides, but also on the stability of Earth’s tilt.

Earth’s orbital tilt of about 23 and a half degrees as it goes around the Sun causes the seasons. The axis precesses, describing an aparent circle roughly every 26,000 years, but the amount of tilt (obliquity) stays nearly the same. Because the Moon acts a counterweight, the obliquity varies only between 22.1 degrees and 24.5 degrees over about 41,00 years (we are now at 23.44 degrees and decreasing). Even this orderly variation, called the Milankovitch cycle, is enough to influence our Ice Ages. Imagine the impact on Earth’s climate if there were no Moon, and the obliquity varied chaotically. This is exactly what happens at Mars, where the tiny moons Phobos and Deimos are not massive enough to influence Mars’ tilt.

Public Domain: Apollo 8 Looks at the Moon (NARA/NASA)
Creative Commons License photo credit: pingnews.com

One thing our Moon does have in common with most others is that it orbits the Earth and rotates on its own axis at the same rate. This is called ‘synchronous rotation’ and it occurs because the Moon is not exactly uniform in composition. From the time the Moon formed, the slightly heavier side was attracted to the Earth. Over time, this effect de-spun the Moon until it attained synchronous rotation. The Moon’s gravitational attraction also de-spins the Earth, although much more slowly as the Moon is less massive. As it does so, the Moon moves slightly farther from the Earth (just over 3 cm per year). The Moon is now 1.5 meters farther away that it was when Apollo astronauts went there. Don’t worry, though, by the time the Moon is far enough away to escape, the Sun will have become a red giant and swallowed both Earth and Moon anyway.

What is the shape of the Moon’s trajectory around the Sun? Perhaps not what you’d expect.

So, I encourage every one to watch for the reappearance of the Moon in the evening sky this week, even if you aren’t celebrating a New Year or an ‘Eid. The Earth’s companion gives all of us something to appreciate.