Go stargazing! July edition

July 1, 2008

Creative Commons License photo credit: KingArthur10

Jupiter is up all night long this month. At about 3 am on July 9, the Earth will pass between the Sun and Jupiter. This alignment is called opposition because it puts the Sun and Jupiter on opposite sides of the Earth.

Being opposite the Sun in our sky, Jupiter rises at sunset and sets at sunrise; it is up literally all night long the night of July 8-9 and up virtually all night for the whole month. Jupiter outshines everything else in the night sky this month unless the moon is present.

Jupiter is easy to find this July; it is low in the southeast at dusk or low in the southwest at dawn.

Mars and Saturn are also visable this month. Look west at dusk to find stars in the shape of a backwards question mark.  These form the mane of the constellation Leo the lion.  The point under the question mark is Regulus.

Saturn is to Regulus’ upper left.  On July 1, Mars is to the lower right of Saturn, near Regulus.  Saturn is the brighter of the two; Mars continues to fade each day as Earth pulls away from it. Watch each night as Mars approaches Saturn and passes it on July 10. By the end of July, Mars will be up and to the left of Saturn, and both will be lower to the horizon at dusk. The Moon is near Mars and Saturn on July 6th. Venus is lost in the Sun’s glare, and will remain out of sight through the end of the summer.

International Space Station & Half-Moon & Saturn & Regulus
Creative Commons License photo credit: scyllarides

The brightest star in the sky tonight is Arcturus, which you can find by extending the arc of the Big Dipper’s handle (arc to Arcturus).  Arcturus, the fourth brightest star we see at night, is the brightest star left since the top three are not visible in Houston in July.

The Big Dipper happens to be to the upper left of the North Star at dusk this month.

Scorpius, the Scorpion, is in the southeast at dusk.  Sagittarius, the Archer, known for its teapot asterism, is to Scorpius’ left.

 In the east, look for the enormous Summer Triangle, consisting of the stars Deneb, Vega, and Altair. This triangle is up all night long in July, hence its name.

Of the three Summer Triangle stars, Vega is the brightest (in fact, 5th brightest overall and 2nd only to Arcturus on July nights). However, Deneb actually puts out far more light, despite the fact that Deneb is about 3000 light years away, compared to 25 light years for Vega.  Star brightness depends not only on intrinsic power, but on distance.

Moon Phases in July 2008:

New July 2, 9:19 pm
1st Quarter July 9, 11:34 pm
Full July 18, 2:59 1m
Last Quarter July 25, 1:42 pm
Full Moon
Creative Commons License photo credit: Andréia

At about 3 am on Friday, July 4, 2008, the Earth is at aphelion (greatest distance from the Sun). We were closest to the Sun about six months ago, on January 2.  This serves as an excellent reminder that it’s the Earth’s tilt on its axis, and not its varying distance from the Sun, which causes our seasons.

Astronomers define the average Earth-Sun distance (about 93 million miles) as one astronomical unit, or AU. On July 4, we will have moved out to 1.016 AU, while on January 2 we were at 0.983 AU. This is not enough of a difference to affect how much warmth comes our way; it’s going to stay hot and sticky for a while.

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

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