Go Stargazing! July Edition

Saturn is the only planet visible to the naked eye in the evening skies of July, 2011.  Face south-southwest at dusk, and you’ll see Saturn near a star of similar brightness—Spica in Virgo.  Saturn is a bit to the right of Spica as you face southwest.   The ringed planet remains well placed for evening viewing, and remains in the evening sky until late September 2011.

Hubble Images Suggest Rogue Asteroid Smacked Jupiter
Creative Commons License photo credit: NASA Goddard Photo and Video

Mars and Jupiter are now higher in the pre-dawn sky.  Jupiter, set against a background of very dim stars, dominates the eastern sky at dawn.  Mars is dimmer and much lower in the east northeast.  It has fully emerged from the sun’s glare, and will brighten slightly each morning.  Venus is now out of sight, as it is passing around the far side of the sun from our perspective.

The Big Dipper is above the North Star, with its handle pointing up.  From that handle, you can ‘arc to Arcturus’ and then ‘speed on to Spica’; those stars are in the southwest at dusk.  Leo, the Lion, sets in the west at dusk.

Antares, brightest star of Scorpius, the Scorpion, is in the south, with the ‘teapot’ of Sagittarius rising behind it.  The Summer Triangle has fully risen in the northeast.  The stars of summer now dominate the evening sky.

Moon Phases in July 2011:

New Moon                       July 1, 4:02 p.m.

1st Quarter                     July 8, 9:09 p.m.

Full Moon                        July 15, 3:12 p.m.

Last Quarter                  July 23, 6:48 a.m.

Winter Solstice Lunar Eclipse
Creative Commons License photo credit: Bruce McKay~YSP

The new moon of July 1 partially blocks the sun, but only as seen from the Antarctic.  No one will get to see a total eclipse because the moon’s full shadow, or umbra, passes just below the Earth.

As we celebrate our independence this July 4, Earth will be at aphelion (at its greatest distance from the sun).  The precise time is 10 a.m.  Perihelion, the Earth’s closest approach to the sun, occurs in January.  Earth has perihelion and aphelion because its orbit is not a circle but an ellipse with an eccentricity (out-of-roundness) of about 1.6%.  Such a small variation, however, exerts no significant influence on our seasons, as you can determine for yourself by stepping outside.  The 23.5 degree tilt of Earth’s axis, on the other hand, is a much more dominant effect.  The very high midday sun of July ensures long days and baking heat in Houston and across much of the Northern Hemisphere.

By popular demand, our George Observatory will open to the public Fridays and Saturdays this summer (except July 8, due to a prior booking).  The Discovery Dome, our traveling planetarium, will be set up each of these Fridays to show films throughout the evening.

Go Stargazing! March Edition

Hubble Images Suggest Rogue Asteroid Smacked Jupiter
Creative Commons License photo credit:
NASA Goddard Photo and Video

Jupiter leaves the evening sky this month.  For now, you can still observe it in the west at dusk, where it sets by 8:25 on March 1.  It outshines all stars in the sky, so it’s easy to find.  Face west at dusk and look for the brightest thing there.  However, Jupiter is getting a little lower in the sky each evening.  You should be able to follow it until about the middle of the month.  By month’s end, Jupiter is lost in the sun’s glare.  On April 6, it is directly behind the sun from our perspective.

Mercury emerges from behind the sun this month and appears beside Jupiter before Jupiter fades from view.  On March 15, Mercury is about two degrees to the right of Jupiter as they both set in twilight.  As Jupiter becomes lost in the sun’s glare, Mercury remains visible low in the west at dusk for the rest of the month.

Venus remains a dazzling morning star.  It is getting lower in the sky as the angle between the solar system plane and the horizon gets shallower.  Face southeast at dawn, and you can’t miss it.

Saturn is in the southwest at dawn, getting lower in the southwest by month’s end.  This is because at the end of the month, Earth is about to pass between the sun and Saturn.  This alignment, called opposition, puts Saturn in the sky all night long; it rises in the east at dusk and sets in the west at dawn (the precise opposition date is April 3).  As a result, Saturn is also an evening object, rising in the east by 9:00 p.m. on March 1 and by dusk on the 31.

Mars, just past conjunction with the sun, remains lost in the sun’s glare all month.

A swath of brilliant winter stars continues to dominate evening skies.  These stars are now due south at dusk, but shift to the southwest later in the evening.  Orion, the Hunter, is almost due south.  His two dogs, represented by Sirius and Procyon, are to his left.  Above Orion is Taurus, the Bull with Aldebaran as its eye. Gemini, the Twins, are to Orion’s upper left.  Leo, the Lion, rises in the east.  The Big Dipper has now fully re-entered the evening sky; it is to the right of the North Star with the handle pointing down.

Below Sirius, just above the southern horizon, is a star second only to Sirius in brightness.  This is Canopus, which marks the keel (bottom) of the legendary ship Argo Navis.  Canopus is so far south, in fact, that most Americans never see it.  From the Gulf Coast, however, Canopus does rise.  March and March are the best months to see it in the evening.

Moon Phases in March 2011:

New Moon                              March 4, 2:46 p.m.

1st Quarter                             March 12, 5:45 p.m.

Full Moon                               March 19, 1:10 p.m.

Last Quarter                          March 26, 7:07 a.m.

At 6:21 p.m. CDT on Sunday, March 20, the sun is overhead at the Earth’s equator, giving everyone in the world the same amount of daylight.  This, then is the vernal equinox, the ‘official’ start of spring.  For us, days have been lengthening since December 21; by now daytime is almost as long as the night.  After March 20, daytime is longer than night for us.  For many people, however, wintry weather continues so long as arctic air masses remain in motion across North America, Europe, and Asia.

Winter time
Creative Commons License photo credit: cvanstane

People in the Southern Hemisphere had their longest days back in December; their days have since shortened to be about equal to the night.  After March 20, night is longer than day down there, so this is their autumnal (fall) equinox.

Sunday, March 13, is the second Sunday in March.  Therefore, Daylight Saving Time begins at 2 that morning (1:59:59 is followed by 3:00:00).  Don’t forget to set your clocks forward one hour on Saturday night, March 12!

The Great Planet Race

During July and August 2010, you can watch a great planet race as Venus and Mars close in on and then pass Saturn. Observing this will also give you a sense of how the ancients, even thousands of years ago, could distinguish the planets from the stars and from each other.

Thousands of years ago, the earliest astronomers noticed that they could form patterns with the stars.  They also noticed that these patterns remained the same throughout their lives and even across generations.

In contrast to these “fixed stars,” there were seven “wandering stars.”  Consistent observation revealed that five points of light in the night sky shifted position noticeably from night to night.  After a year, even the slowest of these was clearly “out of place.”   The other two “wanderers” were the sun and the moon.  From the Greek word for “wanderer,” today we call these moving objects planets.  (The sun and moon were thus “planets” until we understood the solar system better.)

Cellarius ptolemaic system

With more careful observation, we can clearly distinguish the planets from one another using only the naked eye.  One of the planets far outshines all the others, and in fact outshines everything in the sky except the sun and moon.  Ancients named this one after the goddess of love and beauty–Aphrodite for the Greeks and Venus for the Romans.  Another planet has a distinctly reddish tint compared to all of the others, whose light is closer to pure white.  This one has therefore borne the names of gods of war, such as the Greek Ares and Roman Mars.

“Jupiter of Smyrna” currently residing
in the Louvre in Paris, France.


An important way to distinguish among the planets was to observe them when several were close together and to note which ones moved faster compared to others.  The fastest moving planet, the one that always passed up the others (unless it appears to stop and head the other way), received the name of the swiftest god.  For the Greeks, this was the messenger god Hermes, for the Romans it was Mercury.  On the other hand, there were two planets so slow that the motion was barely noticeable from night to night, but could be detected over months or years.  These two were considered rulers of heaven since they were the farthest away.  After all, ancients noticed that faraway objects, such as ships sailing at the limit of their vision, seemed to be going slower than similar objects close by.    The very slowest planet is also the dimmest; any other planet at its brightest outshines it.  The second slowest, on the other hand, outshines all stars at night and all planets except Venus.  Thus, the Greeks identified the slowest planet with the former, deposed ruler of heaven (Kronos/Saturn).  The planet which is brilliant despite its great slowness and distance was the true ruler of heaven (Zeus/Jupiter).

Now, you can go outside and make these same types of observations. In July 2010, face west at dusk to find three planets.  Venus is by far the brightest, outshining all the planets and stars.  Mars and Saturn are to the left and  slightly higher in the sky.  Although they aren’t nearly as bright as Venus, Mars and Saturn easily outshine all stars in their immediate vicinity and are therefore just as noticeable.  Mars is between Venus and Saturn and slightly below a line joining those two.

During the rest of July, Mars will close the gap on Saturn, until by July 31 it appears less then 2 degrees under the ringed planet.  Meanwhile, Venus will have closed to less than 8 degrees to the right of the pair.  Keep watching in August as Mars pulls ahead of Saturn while Venus begins to form a compact triangle with them both.  Venus is less than 3 degrees below Saturn on August 8 as it continues to gain on Mars.  Finally, Venus catches up to Mars and is less than three degrees below it on August 19.  By the end of August, the three planets have reversed their order; Venus has become the leftmost of the three, while Saturn will be on the right.

As you keep observing, you will also notice that Venus begins to slow down a little once it has “won” the race.  Also, it begins to move off of the imaginary line joining Mars and Saturn.  This is due to the geometry of our solar system.  Mars and Saturn are traveling slower than earth on their outer orbits.  Thus, Earth is leaving them behind and will pass on the far side of the Sun from them.  That’s why we see Saturn (in September) and Mars (in December) drop into the Sun glare.

Venus, on the other hand, is orbiting inside Earth’s orbit and is therefore going faster than Earth.  We’re seeing Venus come out from behind the Sun, and then around to our side of the Sun.  That’s why Venus seems to slow down–it’s starting to move towards the Earth.  Once Venus is on our side of the Sun, we see it move backwards (or retrograde) against the background stars.  In October, when Venus “laps” the Earth, we can’t see it at all.

Viewers with perfectly clear horizons who observe right at dusk may also glimpse Mercury.  The elusive Messenger is to the lower right of the other three planets and roughly in line with them.  Mercury won’t catch up to the other three however.  By August, Mercury will have come around to our side of the Sun, so we’ll see it head back towards the Sun’s glare before it aligns with Saturn, Mars, or Venus.

The King of Planets, Jupiter, sits this one out.  he makes a grand entry into the eastern sky at dusk this September, is up all night long on September 21, and dominates the evening sky throughout the fall.


Science Doesn’t Sleep (7.8.08)

from-airplane-greenland-12
Creative Commons License photo credit: william.ward

So here’s what went down after you logged off.

Are melting glaciers causing sea levels to rise? A team from Utrecht University says no. A team from Woods Hole Oceanographic Institute is exploring that issue this month – check back here for updates from Chris Linder.

And you thought the Sun was harsh – “O” stars in the Rosetta Nebula “can be a hundred times the size and over a thousand times brighter” – and they destroy planets.  

Despite the fact that scientists have traditionally been wary of Wikipedia – which relies on the “wisdom of crowds” – a new Gene Wiki is being developed to “describe the relationship and functions of all human genes.”

Ancient river camps show humans in Paris almost 10,000 years ago.

Researchers have developed a way to trick kidney cancer cells into killing themselves.  

The Chronicle has a new space blogCosmo.Sphere - written by a UT astronomer, a NASA vehicle systems engineer and a long-time amateur astronomer.