Go Stargazing! July edition

Jupiter becomes a late evening object by the end of the month.  It rises in the southeast just after 11 p.m. on July 1, although you may need to wait awhile for it to clear trees or buildings in that direction.  By month’s end, Jupiter rises at 9 p.m. — in late twilight.  Early risers can still see Jupiter in the southwest before dawn.  Next month, Jupiter is in the sky literally all night long.  Remember, Jupiter outshines everything in the sky except the Sun, the Moon, and Venus, so if you’re looking in the right direction, you can’t miss it.

Venus is a dazzling morning star this month.  Look east right as day begins to break for the brightest thing unless the Moon is nearby.  Venus remains the ‘morning star’ for the rest of 2009.  Mars is a little higher in the east at dawn than it has been.  Still, it remains fairly dim.  Look for Mars above Venus and to its right.  This is quite a mismatched pair; Venus is about 100 times brighter than Mars.

Saturn portrait
Creative Commons License photo credit: Elsie esq.

Saturn remains well placed in the evening sky this month.  Look for it in the west at dusk.  If you have seen Saturn through a telescope this year, you may have noticed how much thinner the rings appear now than in years past.  This is because Earth is beginning to align with Saturn’s ring plane, making the rings appear edge-on from our perspective.  On September 4, the Earth is exactly in Saturn’s ring plane, and the rings actually vanish from view!  It turns out, though, that Saturn is too close to the Sun in our sky on that date; the Earth will be about to pass on the far side of the Sun from Saturn.  No one can get a good look at Saturn this September.  However, we can still watch through our telescopes as Saturn’s rings appear thinner and thinner throughout July and August.

Saturn’s moons orbit in the same plane as its rings.  Since we ordinarily have a perspective looking over one of Saturn’s poles, moons such as Titan and Rhea can usually appear above or below Saturn as well as to its right or left in a telescopic image.  These moons are not normally blocked by Saturn.  That changes, however, when Earth aligns with Saturn’s ring plane.  Now that we’re seeing the entire system edgewise, we’re beginning to see Saturn’s moons pass in front of and behind Saturn’s disk.  The passage of a moon in front of a planet’s disk is a transit, while an occultation occurs when a planet’s disk blocks a moon.  When a moon transits, we can often see its shadow on the planet’s disk.  Here are some upcoming events for Saturn and Titan as seen from Houston:

7/9        Titan is partly occulted (blocked) by Saturn until 9:30 pm.

7/17      Titan is already in transit as night falls; it leaves the Sun’s disk between 9:45 and 10:20. (Titan appears as a disk and not a point, so it takes some time to move all of the way off Saturn’s disk.  Saturn sets by 11:15.

7/25      Titan is occulted by Saturn.

8/2        Titan is in transit from dusk until Saturn sets.  Titan’s shadow appears on Saturn’s disk at 9:30.

8/10      Titan occulted by Saturn

8/18      Titan transits Saturn.

By August 18, however, Saturn is so close to the Sun in our sky that it is only about five degrees high during late twilight and sets before night completely falls.

M42 Orion
Creative Commons License photo credit: makelessnoise

Look high in the west at dusk for stars in the shape of a backwards question mark, with a right triangle to the left of that.  These stars are in Leo, the Lion.  Saturn is under the ‘right angle’ in that right triangle.  The Big Dipper is high in the northwest on summer evenings.  From the Big Dipper’s handle, you can ‘arc to Arcturus.’  Arcturus, in the west at dusk, is the fourth brightest star we ever see at night and will be the brightest star in our night skies during all of July. Continuing the curve of the Big Dipper’s handle past Arcturus, you can ‘speed on to Spica,’ a star lower in the southwest at dusk.  Spica is a stalk of wheat held by Virgo, the Virgin, who represents the harvest goddess.

In the south as night falls is Antares in Scorpius, the Scorpion.  This is a red super giant star about 700 times as wide across as our Sun.  To the Scorpion’s left, look for eight stars in the shape of a teapot.  These stars are the bow and arrow of Sagittarius, the Archer.  In the east, the Summer Triangle dominates the evening sky.  The Triangle is up all night long until mid-August.  Vega is the brightest of the triangle’s three stars, followed by Altair in Aquila and Deneb in Cygnus.

Moon Phases in July 2009:

Full                                   July 7, 4:21 am
Last Quarter                     July 15, 4:53 am
New                                  July 21, 9:34 pm
1st Quarter                       July 28, 4:59 pm

Eclipsed? Not totally.
Creative Commons License photo credit: James Jordan

The New Moon of July 21 lines up well enough with the Earth and Sun to cast its shadow on the Earth.  This causes a total solar eclipse.  The Moon’s shadow first encounters the Earth just north of Mumbai in India, so that’s where the path of totality begins.  From there, the shadow moves across Bhutan and then southern China, including Shanghai.  The shadow then ends up over the Pacific Ocean and leaves Earth before ever again reaching land.  The only part of the US anywhere close to this path is Hawaii, which experiences a partial eclipse.  This is mostly an event for Asia, where the date will be July 22.

The next total solar eclipse visible in the USA will occur August 21, 2017.

The Full Moon of July 7 almost enters the Earth’s shadow.  It does skirt the edge of the penumbra, in which the Earth partially blocks the Sun.  The resulting penumbral eclipse is scarcely noticeable at all, however.

At 3 a.m. on Friday, July 3, Earth is as far as possible from the Sun (i.e., at aphelion).  Planetary orbits are not perfect circles but ellipses.  Thus, Earth does not remain at the same distance from the Sun throughout its orbit, but gets slightly closer in January and slightly farther in July.  The difference is only about 3.4%, however—not enough to affect our seasons.  The change in seasons is due to the Earth’s tilt on its axis, not the distance from the sun.