Selected Sky Events for August 2012
by Robert C. Victor
Events for August 7, 13-14, and 20-21 are illustrated below. For illustrations of additional events, refer to the Abrams Planetarium Sky Calendar. Subscription information is available at www.pa.msu.edu/abrams/skycalendar. Here are the selected sky events for August, along with some questions to prompt discussions with students:
- Wed. Aug. 1, one hour after sunset: Saturn and Spica are now within 4.5°, forming a nearly isosceles triangle with Mars, within 8° to their west. On Aug. 3, Saturn passes conjunction with Spica in celestial longitude for the third and last time during this apparition. They reach least separation of 4° 27’ apart on Aug. 6 and won’t appear that close again until 2041.
- Tues. Aug. 7, one hour after sunset: Mars now pulls to within 4.8° of Saturn and within 4.2° of Spica. Saturn is within 4.5° of Spica. All three sides of the triangle remain less than 5° long tonight through August 20. Image courtesy of Abrams Planetarium. Subscriptions to the sky calendar ar $11.00 per year, starting anytime, from Sky Calendar, Abrams Planetarium, Michigan State University, 755 Science Rd, East Lansing, MI 48824 or online.
- Sat. Aug. 11, 1-1/2 hours before sunrise: Here is a beautiful gathering of the waning crescent Moon with bright Jupiter (mag. –2.2), Aldebaran, and the Hyades star cluster. Jupiter is 4° to 5° lower left of the Moon from mid-U.S. (The Moon occults Jupiter by 12 noon from Hawaii). Aldebaran is just over 5° S of Jupiter. There is a triple conjunction of Jupiter and Aldebaran in 2012-2013, in each case the planet going N of the star. Least separations occur on July 30 and December 11 (4.7°), and on March 18, 2013 (5.1° apart).
Also visible this morning, but not shown, is Venus, now 22° lower left of Jupiter. Within a half an hour, watch for Mercury (mag. +0.9), rising 28° lower left of Venus, and Procyon, rising 15° right of Mercury.
- Mon. Aug. 13, one hour before sunrise: Venus appears within 4° E of the waning crescent Moon from mid-U.S. This is a perfect day to use the Moon to follow Venus long into daylight hours. The Moon occults Venus in afternoon from most of North America, soon after 1 p.m. PDT from U.S. West Coast but just before moonset from E coast. For a map and timetable of disappearance and reappearance, visit www.lunar-occultations.com/iota/planets/0813venus.htm. To convert UT to your local time, use www.lunar-occultations.com/iota/utc.htm. Image courtesy of Abrams Planetarium. Subscriptions to the sky calendar ar $11.00 per year, starting anytime, from Sky Calendar, Abrams Planetarium, Michigan State University, 755 Science Rd, East Lansing, MI 48824 or online.
In morning twilight, Mercury (mag. +0.5) is 28° lower left of Venus. (See Aug. 15 and 16.) Also visible is Jupiter (mag. –2.2), 23° upper right of Venus. Keep watch low in ESE for the rising of the Dog Star Sirius, 26° to the right and a little lower from Procyon, the “before the dog” star announcing the imminent rising of Sirius.
- Tues. Aug. 14, one hour after sunset: A striking, nearly straight-line arrangement of three bright objects: Saturn (mag. +0.8) highest, with Mars of mag. +1.1 located 2.7 degrees below Saturn on their evening of least separation, and finally +1.0-mag. Spica 1.8° below Mars. Mars and Spica were closest last night, 1.7° apart. Tonight’s distances in light travel time: Mars 14 minutes, Saturn 85 minutes (6 times as far away as Mars), and Spica 260 years.
- Wed. Aug.15, one hour before sunrise: This morning Venus (mag. –4.4) reaches greatest elongation, 46° W of Sun. Through a telescope it appears as a half-illuminated disk, 0.4 arcminute across. Far to its lower left is a beautiful crescent Moon, with +0.1-mag. Mercury 8° or 9° to its lower left.
- Thurs. Aug. 16, 45 minutes before sunrise: This morning is Mercury’s turn to reach greatest elongation, in this case just 19° W of the Sun. Also on this date, Mercury reaches its least distance from Venus during this apparition, 27°. Binoculars might help you spot the thin old crescent Moon, rising about 6° below and a little right of Mercury. The Sun is about 8° below the ENE horizon, 19° lower left of Mercury.
Visualize this: If you could park yourself in space north (upper left) of the Sun, so you could watch from “above” the solar system to see the planets revolve around the Sun, in which direction would they go? Recall that Venus passed inferior conjunction, between Earth and Sun, during the transit on June 5, and Mercury passed inferior conjunction, but without a transit, on July 28. Both planets pass greatest elongation just a day apart on Aug. 15-16, and are therefore now heading around toward the far side of the Sun. Earth is revolving around the Sun also, but Mercury and Venus, on the inside tracks, are going faster. From the vantage point north of the solar system, the planets revolve counterclockwise around the Sun. Mercury will reach superior conjunction on Sept. 10, but Venus won’t do so until March 2013.
Here is another question to ponder: Venus reaches its greatest brilliance about five weeks before and after inferior conjunction, while it’s in crescent phase about one-fourth full, on the nearer side (but not the closest point) of its orbit. But Mercury is brightest close to superior conjunction, when it is near its maximum distance from Earth. So now, even though both planets are near greatest elongation west of the Sun, Venus is fading and Mercury is getting brighter. Can you explain why?
- Mon. Aug. 20, one hour after sunset: The triangle Mars-Saturn-Spica, for the last time, is still less than 5° on each side. The shortest side is Mars-Saturn, 4.1° long. Look early to catch the Moon about 12° lower right of Spica.
- Tues. Aug. 21, one hour after sunset: Here’s a compact gathering of the crescent Moon, two planets, and a star, all within a 6.5° field. Can you fit all four bodies within the field of view of 7-power binoculars? Of the three bright stellar bodies, Mars (mag. +1.2), Saturn (mag. +0.8) and Spica (+1.0), which do you think will still be visible evenings in October? (In fact, it will still be visible at dusk at year’s end.)
I hope you have enjoyed the selection of sky events for August. In the rest of 2012, we’ll see spectacular conjunctions of the waning crescent Moon and Venus before dawn on Sept. 12, Oct. 12, Nov. 11, and Dec. 11. Jupiter will appear near the Moon on the mornings of Sept. 8, Oct. 5 and 6, Nov. 1 and 2, and evenings of Nov. 1, 28, and Dec. 25. Venus and Saturn will appear in a close pairing on the mornings of Nov. 26 and 27. All these and many more events will be illustrated in future issues of Sky Calendar.
Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs, CA.
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