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01 Jul 2013

July Astronomy: This Month, Add a Little Spica to Your Saturn

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July Astronomy: This Month, Add a Little Spica to Your Saturn Robert D. Miller

In July 2013, the brilliant evening “star” Venus gleams low in evening twilight, drifting from west-northwest to west as month progresses. Valley residents living near the mountains (in downtown Palm Springs, for example) will have to seek out a location where mountains don’t block the view.

On our evening all-sky chart (above), planets are plotted for each day when the Sun has sunk to 9 degrees below the horizon, at mid-twilight. By then, the two naked-eye planets and eight stars of first magnitude or brighter plotted on the chart are easily visible, except for Pollux and Regulus sinking in the twilight glow. In July, from the Coachella Valley, mid-twilight occurs about 45 minutes after sunset.

Planet positions are represented by a separate dot for each date, with positions for each Monday in July (1, 8, 15, 22, 29), represented by a larger dot and labeled. We find Venus and Regulus in the western sky this month. Rotate the chart until the portion of the horizon circle nearest to the pair is at bottom, and you’ll see the planet and star depicted at the same orientation as they appear in the west to west-northwest sky: Regulus is 25 degrees to the upper left of Venus on July 1, and 12 degrees to the lower right of Venus on July 31.

Another planet is present on July evenings: Saturn, tracking from south to southwest in mid-twilight as July progresses. Notice the first-magnitude star Spica 12 degrees to the west (lower right) of brighter Saturn all month, with the blue-white twinkling star preceding the steady yellowish planet as both objects go westward across the sky.

The brightest star in July’s evening sky is Arcturus, high in south-southwest to west-southwest, above Saturn and Spica, and forming a large triangle with them. When the Big Dipper becomes visible, you can “follow the arc (of the handle) to Arcturus and drive a spike to Spica.”

Next after Arcturus in brilliance is Vega, climbing high in the east-northeast. Compare the contrasting colors of these two stars! To Vega’s lower left is Deneb, and to their lower right is Altair, completing the Summer Triangle. Climbing in the south-southeast to south is reddish Antares, heart of the Scorpion.

Find Regulus, heart of Leo, the Lion, within 5 degrees of Venus during July 18-25. They appear closest on the evenings of July 22 (1.2 degrees apart), and July 23 (1.3 degrees). Sinking lower nightly, Regulus will pass on the far side of the Sun on Aug. 22.

Follow the Moon

During July 10-23, the moon is above the horizon in evening mid-twilight. Follow it nightly as it waxes, or grows, from a thin crescent on July 10, through first quarter (half full) on July 15, to full on July 22. The moon passes, in order, Venus on July 10, Regulus on the 11th, Spica on the 15th, Saturn on the 16th, and Antares on the night of July 18.

I recommend subscribing to the Abrams Planetarium’s Sky Calendar; it illustrates the moon’s changing position against background stars in July, and the changing arrangements of Venus-Regulus at dusk and Mars-Jupiter-Mercury at dawn. For information on subscribing, visit www.pa.msu.edu/abrams/SkyCalendar.

July Moonrise Watch

Full moon at 11:16 a.m. on Monday, July 22, occurs 22 hours after the moon’s third-closest approach of the year, 222,700 miles from Earth.

From the Coachella Valley that evening, the moon rises 17 degrees south of east at 7:48 p.m., about six minutes before sunset. Does the full moon at rising seem unusually large this month? The moon at rising or setting always seems large (the “moon illusion”), even when it is at its most distant from Earth.

For those who enjoy watching or photographing the Moon’s big reddened disk coming up over distant mountains, here are moonrise times for the Coachella Valley for a week, starting at the full moon. Times are when the moon’s disk would just start to appear over an ideal, flat horizon. In practice, our mountainous surroundings may delay risings and hasten settings by several minutes.

Monday, July 22: 7:48 p.m.—17 degrees south of east (full moon)

Tuesday, July 23: 8:32 p.m.—12 degrees south of east (97 percent)

Wednesday, July 24: 9:12 p.m.—6 degrees south of east (92 percent)

Thursday, July 25: 9:50 p.m.—due east (84 percent)

Friday, July 26: 10:25 p.m.—6 degrees north of east (75 percent)

Saturday, July 27: 11:01 p.m.—11 degrees north of east (65 percent)

Sunday, July 28: 11:37 p.m.—15 degrees north of east (54 percent)

Dawn

Our all-sky chart for morning mid-twilight (below) depicts the sky about 45 minutes before sunrise in Southern California. The Summer Triangle of Vega-Deneb-Altair is high in the western sky at dawn, sinking lower as month progresses. During all of July, the Summer Triangle is up all night.

Bright Jupiter emerges by end of first week, low in the east-north east to the lower left of faint (magnitude +1.6) Mars. To their upper left, find bright Capella, the “Mother Goat” star, in the northeast, higher as month progresses. To upper right of Jupiter is reddish Aldebaran, eye of Taurus the Bull, with the compact Pleiades star cluster, or Seven Sisters (not shown), 14 degrees higher. Fomalhaut, mouth of the Southern Fish, drifts from south to south-southwest. By July’s fourth week, Betelgeuse and Rigel, shoulder and foot of Orion, the Hunter, rise from the dawn glow into the eastern sky. (Orion’s belt, a nearly vertical line of three stars midway between them, isn’t plotted.)

Faint reddish Mars and bright yellowish Jupiter appear no more than 5 degrees apart July 11 through Aug. 1, and as close as 0.8 degrees apart on July 22. By July 25, Mercury has emerged as a first-magnitude “star” to their lower left, and brightens to magnitude 0 by month’s end.

The waning crescent moon in the morning sky passes near the Pleiades star cluster on July 4 and 31, near Aldebaran on July 5 and Aug. 1, near Mars and Jupiter on July 6 and Aug. 3-4, and near Mercury on Aug. 5.

Star Parties

The Astronomical Society of the Desert hosts free public sky-watching sessions at Sawmill Trailhead this summer on Saturday evenings, July 6, Aug. 3 and Sept. 7. For more information and directions, visit www.astrorx.org and www.astrorx.org/Sawmill%20Directions.htm.

Robert C. Victor was a staff astronomer at the Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. Robert D. Miller did graduate work in planetarium science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

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