Seeing Stars with James Wooten: March 2013

Jupiter is almost overhead at dusk, but now a little toward the west. Face high in the west at dusk and look for the brightest thing there (unless the Moon is also there), as Jupiter outshines all stars we ever see at night.

Saturn remains in the morning sky this month.  Look for it in the south/southwest at dawn.

Venus and Mars are on the far side of the Sun and out of sight this month. Venus passes behind the Sun (at superior conjunction) on March 28.

Sky Map: March 2013

Brilliant winter stars shift toward the southwest during March. Dazzling Orion is almost due south at dusk. His three-starred belt is halfway between reddish Betelgeuse and bluish Rigel. Orion’s belt points up to Aldebaran in Taurus the Bull. This winter and spring the Bull also contains Jupiter.

To Orion’s upper left are the twin stars Castor and Pollux, marking the heads of Gemini, the Twins. You can find Sirius, the brightest star we ever see at night, by drawing a line from Orion’s belt towards the horizon. To Orion’s left, about level with Betelgeuse, is Procyon, the Little Dog Star.

From Sirius, look a little bit to the right and then straight down to the horizon. If your southern horizon is clear of clouds and tall earthly obstacles, you’ll see Canopus, the second-brightest star ever visible at night. This star is so far south that most Americans never see it and many star maps made in the USA omit it. (You must be south of 37 degrees north — the latitude of the USA’s Four Corners — for Canopus to rise). As you view Canopus, keep in mind that the sky we see depends on our latitude as well as on time of year and time of night.

Joining the winter stars are stars of spring rising in the east.  Look for Leo, the Lion at dusk. Later in the evening, extend the Big Dipper’s handle to ‘Arc to Arcturus’ and then ‘speed on to Spica’; these stars rise at about 10 p.m. in early March but by 9 p.m. on the 31st.

March 2013 evening skies feature an extra special object — comet Pan-STARRS, our first naked-eye comet since Hale-Bopp back in 1997!  Observers south of the equator have already been observing Pan-STARRS, but the comet has been invisible to us because it has been south of the Sun in our sky. That is beginning to change as Pan-STARRS nears its closest approach to the Sun late on March 9.

Like all comets, Pan-STARRS will be at its brightest as it comes closest to the Sun.  At the same time, Pan-STARRS will be coming up through the plane where the planets orbit and thus will be much easier for us to see in mid-March. You can start looking in western twilight as early as March 7 if you have a low, unobstructed horizon. The comet may be slightly easier to see on March 12 and 13, when the crescent Moon is nearby. Once Pan-STARRS appears in the western dusk sky, it shifts towards the north (to the right as you face west) each night, until it fades and returns to the Sun’s glare in April.

As always, scientists are unsure how bright Pan-STARRS will get.  It now seems that it won’t be as spectacular as was Hale-Bopp in 1997. However, southern observers are seeing it naked-eye, and so should we. The comet could be about as bright as average stars such as those in the Big Dipper, but may be dim enough that you need a dark site to see it, especially once the Moon gets bigger.

Ultimately, though, we’ll have to wait and see. Sky and Telescope has a helpful finder chart here.

Moon Phases in March 2013:
Last Quarter                  March 4, 3:54 pm
New                               March 11, 2:53 pm
1st Quarter                    March 19, 12:26 pm
Full                                March 27, 4:29 am

At 6:01 a.m. on Wednesday, March 20, the Sun is directly overhead at the equator.  This therefore is the vernal (spring) equinox, a day when everyone on Earth has the same amount of daylight. After this date, our gradually lengthening days become longer than our nights, and we go into springtime. South of the equator, days have been shortening.  For them, this equinox marks the point when night becomes longer than day, and the onset of autumn.

Sunday, March 10, is the second Sunday of this month. Therefore, Daylight Saving Time begins at 2 a.m. this morning (the time officially goes from 1:59 to 3 a.m.)  Don’t forget to spring forward by advancing all clocks one hour on Saturday night, March 9!

On most clear Saturday nights at the George Observatory, you can hear me do live star tours on the observation deck with a green laser pointer.  If you’re there, listen for my announcement. This Spring Break, the George will be open to the public on Tuesday night, March 12, and Thursday night, March 14.

To enjoy the stars in any weather from the comfort of the HMNS Planetarium, click here for a full schedule.

See Comet Hartley 2!

Perhaps you were able to observe Comet McNaught this past June and July.  In case you missed that comet, however, another brighter than average comet has approached the Earth this October. Comet Hartley 2 is now visible in binoculars, and could be a naked-eye object around the time of its closest approach to Earth on October 20. As its name indicates, this is the second comet found by Malcolm Hartley. He discovered this comet in 1986 at the Siding Spring Observatory in Australia. Comet Hartley 2 is a small object only about 1.5km across. It orbits the sun once in about 6.5 years, attaining a maximum (aphelion) distance from the sun of about 5.87 AU (beyond Jupiter’s orbit) and a minimum (perihelion) distance of about 1.05 AU (just beyond Earth’s orbit). This orbit places it in the Jupiter family of comets, which orbit in the same direction as the planets with periods of less than 20 years.   In fact, there is evidence that interaction with Jupiter has shortened Hartley 2′s orbital period from over 9 years to the present 6.5.

In October 2010, Hartley 2 comes to perihelion while Earth is on the same side of the sun. This brings Hartley 2 within 11.2 million km of the Earth, close enough to make it visible in our skies—possibly even to the naked eye.  Many comets come inside the Earth’s orbit as they approach the sun, which means we look more or less in the sun’s direction when seeing them at their brightest.  Hartley 2, on the other hand, has a perihelion just outside Earth’s orbit, so Earth is passing roughly between the sun and the comet this month.  Therefore, we can observe it while looking away from the sun in our sky. 

 Halley’s Comet with tail

Comets are made of ice and dust and are often called ‘dirty snowballs.’  We believe they are left over from the formation of the solar system.  As comets approach the sun, ice changes into gas and dust embedded in 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 away from the comet’s head, forming tails pointed away from the sun.  Comets have bluish gas tails and yellowish dust tails.   Since Earth is passing more or less between the sun and Hartley 2, however, its tails will be mostly oriented away from us and foreshortened from our point of view. 

Hartley 2′s coma is now quite large in our sky, so you should look for a fuzzy area, perhaps bigger than the full moon, not a single point of light. The total brightness of the comet is about that of the dimmest stars visible, so the farther you can get from city lights, the better.  The large coma means that Hartley 2′s brightness is diffused over a large area, and therefore the comet may look dimmer than its total brightness would suggest.  Averted vision, which involves looking slightly away from the comet’s actual position, may help you see Hartley 2 if it is at the threshold of visibility.   If you choose a viewing site far from big cities and a night with no moon, you may see the comet with the unaided eye.  The extended coma has a soft, diffuse look comparable to the Milky Way band.  

October 20, the day of closest approach to Earth, has a large waning gibbous moon approaching its full phase on October 22.  Before the full moon, it may be easier to spot Hartley 2 at dawn, after the moon has set.  Or, you can look towards the end of the month, with the moon is at Last Quarter.   Here  is a chart showing Hartley 2′s position through November 3, 2010.  (Note that the dates given are in Universal Time, which corresponds to the previous evening for us.)  Of the constellations shown, Perseus rises in the northeast at dusk in mid-October, while Auriga comes up at about 9:30, also in the northeast.  Gemini, the Twins, rises closer to midnight. Keep in mind that predictions for a comet’s brightness are just that–predictions.  many comets appear significantly brighter or dimmer than expected. 

 Tempel 1 as photographed by Deep Impact

Amateur astronomers who get out and observe this comet won’t be alone in observing Hartley 2.  NASA has retargeted its Deep Impact spacecraft to fly past Hartley 2 on November 4, 2010.  Back in 2005, NASA used Deep Impact to study comet Tempel 1.  In that mission, scientists released a probe to impact Tempel 1 and study the material released.  Deep Impact will simply fly by Hartley 2, however, taking advantage of this opportunity to study yet another comet up close.

With its short orbital period, Hartley 2 should return for its next perihelion near April 20, 2017.  Earth, however, will not be on the same side of the sun as Hartley 2 in 2017, so the comet will be much dimmer in our sky.  Hopefully, our skies will cooperate, and Hartley 2 will brighten as expected or even more, and we’ll all get to appreciate a fascinating sight in the fall 2010 sky.

Early Risers: You’re In For A Treat! June brings Blazing Comet & Lunar Eclipse

Are you an early riser and up before the crack of dawn?  If so, I encourage you to look up as you pick up that morning paper as there are two special treats in the June 2010 morning sky.

Comet McNaught
Creative Commons License photo credit: c.j.b

In January 2007, a brilliant comet, known as Comet McNaught dazzled observers in the Southern Hemisphere. Houstonians missed out on it, though, because of cloudy weather in our area during the brief time that comet was well placed for us. Now, in 2010, a different comet McNaught is becoming visible in our sky at dawn.

Robert H. McNaught, an astronomer at the Australian National Observatory, discovered this comet on September 9, 2009, using a telescope at Australia’s Sliding Spring Observatory.  McNaught, a prolific discoverer of comets, has discovered 44 comets (including this one) and is a co-discoverer of 12 others, for a total of 56.  This comet’s formal designation is C/2009 R1, where ‘C’ indicates a long period comet and ‘R’ indicates the time of year it was discovered.

Comet McNaught, though, is more than a ‘long-period’ comet.  Astronomers have determined that its eccentricity is greater than 1, meaning that its orbit has the shape of a hyperbola.  A hyperbolic orbit is the trajectory of a comet that passes near the sun once and never returns.  Once McNaught recedes from view, we’ll never see it again.

A hyperbolic orbit also means that McNaught has never been in the inner solar system before.  This challenges astronomers who want to predict how it will behave and just how bright it will become in our skies.  Already, McNaught is brighter than expected; many expect McNaught to become a naked-eye object by month’s end, especially for those able to observe at a dark site far from light pollution.  McNaught is now easily observable in binoculars.

This is a chart from Sky and Telescope, showing the path of Comet McNaught against the background stars.  Keep in mind that in June, the stars in this map rise in the northeast just before dawn.  McNaught continues to approach the sun until reaching perihelion on July 2, so we expect it to brighten until that date.  Unfortunately, a comet near perihelion is generally also close to the sun in our sky, and this comet is no exception.  Therefore, McNaught will also get harder to see as it brightens towards the end of the month.   After perihelion, McNaught is poorly placed for observers in the Northern Hemisphere.

17-08-2008 lunar eclipes
Creative Commons License photo credit: emrank

If you’re looking for the comet on Saturday morning, June 26, you might as well turn around and watch the moon set in partial eclipse.  Since the Moon is not precisely aligned with the Earth this time, it will not enter fully into the Earth’s shadow; it goes a little less than halfway in instead.  Still, from 5:17 a.m. until moonset at 6:25 a.m., you’ll notice a chunk of the moon’s upper right side missing.  (Actually, its the northern limb of the Moon that passes through the shadow.  The Moon’s northern limb is on the right as the Moon sets.)  The Moon is only about 10 degrees high when the eclipse starts, so you’ll need a southwest horizon clear of tall trees and buildings.  Note that the eclipse is still in progress at moonset; we will see less than half of it.  Folks far to our west will see a much longer event.

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.