StarWatch: Moravian College Astronomy

StarWatch for the greater Lehigh Valley

JUNE 2013

JUNE STAR MAP | MOON PHASE CALENDAR | STARWATCH INDEX | NIGHT SKY NOTEBOOK

Print Large Sky Charts For 10 p.m. EDT: NORTH | EAST | SOUTH | WEST | ZENITH

Current Solar X-rays:
Current Geomagnetic Field:

Status

Current Moon Phase

876 JUNE 2, 2013: Inferiors Guard the West: During most of June, the action focuses along the western horizon after sundown. There, as the month opens, will be found the two inferior planets of the solar system, Mercury and Venus. The word “inferior” has nothing to do with their status among planets because Venus not only gets closest to the Earth (22 million miles), but it is the third brightest object in the sky after the sun and the moon. Conversely, Mercury, because of its closeness to the sun, is probably the most elusive of the five classical planets, making it my favorite to spot, especially with the unaided eye. “Inferior” simply refers to a position lower to or sunward of the Earth. To view the two inferiors, an unobstructed western horizon will be necessary. Binoculars will help to define them more clearly, especially in a bright western sky. June commences with Mercury running about five degrees ahead of Venus, 45 minutes after sunset, in the WNW. Venus will only be five degrees above the horizon at the time, but because of its brightness, it should pose no difficulty in viewing. During the week of June 9th, Mercury reaches its greatest angle of elongation east of the sun, so that by the end of the week, it appears to slow in its forward motion and move back towards Venus. By week’s end, the two are only three degrees apart, and a comfortable seven degrees above the WNW horizon, 45 minutes after sundown. By summer solstice, June 21st, Mercury has slipped below Venus and is again five degrees above the horizon, three quarters of an hour after sunset. The difficulty with Mercury is that it has become much dimmer, about as luminous as Alioth, the brightest star of the Big Dipper. That is faint for something hugging the horizon, and binoculars will be needed to see Mercury. All the while, Venus remains brilliant. By June’s end Mercury is gone, and only Venus guards the west as it will do for the rest of the summer and autumn.

877 JUNE 9, 2013: Planetary Dance Continues: The astronomical story of this week still focuses on the WNW as Mercury and Venus play in the bright twilight of late spring evenings. Mercury remains in the lead all week, but if you watch it from night to night, it will begin to slow in its motion as it shortly begins its trek towards the sun. It is farthest from the sun on June 24 when it is at its greatest angular distance east of the sun, but its altitude and brightness will have decreased substantially by that time. Even during the week of June 9, binoculars are strongly recommended to see Mercury clearly. Go outdoors about 45 minutes after sundown and look towards the WNW in the same area of the sky as sunset. The most important consideration will be an unhindered western horizon. Venus should be easily seen with just the eye, even during hazy conditions. During the first few days of the week, view Venus using binoculars. Then scan less than one full field of view to the left and above Venus to spot much fainter Mercury. Monday and Tuesday, June 10/11, bring a thin waxing crescent moon with plenty of earthshine to add luster to the dimming twilight hour. The moon will be to the left of Mercury and Venus. As the week unfolds, Venus and Mercury will be closing on each other. Mercury will appear to be circling counterclockwise around Venus by the end of the week. The week of June 16 positions Venus and Mercury less than three degrees apart, and this close twirl continues through the summer solstice, June 21, as Mercury dips below Venus and heads towards the horizon. On June 19, both planets are nearly horizonal to the horizon. Mercury will be to the left of Venus and less than two degrees apart, an easy view through binoculars, especially because of Venus’ brightness. By the onset of the week of June 23, Mercury exits the scene leaving only Venus as the sole guardian of the west after sundown.

878 JUNE 16, 2013: Follow the Waxing Moon: There are lots of “fun things” happening in the sky this week. Look for the first quarter moon on Sunday, June 16. The dark terminator, which divides day from night, will look like a straight line. Since the moon is waxing or growing, the terminator at present represents the sunrise location on the lunar surface. If you scan the moon’s surface with binoculars or a telescope, it is along the terminator where you will observe the greatest amount of detail, since it is here that shadows are at their longest. Then notice bright Spica, ahead and to the left of the moon. The moon is headed towards the principal star of Virgo the Virgin and will be located just to the left of this blue-white supergiant by the 18th. The moon and Spica will make a fine pair in binoculars, but you will notice that the moon’s terminator has changed to a bulbous appearance. The waxing moon is moving through its gibbous phase and has become considerably brighter, easily casting shadows on the ground. The following evening the moon is below and to the left of another “star,” just slightly brighter than Spica. A small telescope or even higher powered binoculars held with steady hands will reveal that this is no ordinary star. It has rings. You’ve just discovered the planet Saturn, which will be steadily moving across the western sky for the next several months before disappearing in bright twilight at the very beginning of autumn. By Friday, June 21, the moon stands above and to the left of the red supergiant Antares in Scorpius the Scorpion, just about as low as it can get in the sky. June 21 at 1:04 p.m., EDT also marks the summer solstice or sun standstill. In the United Kingdom this event is also termed the midsummer sun. The sun can go no higher in the sky for most of the Northern Hemisphere. It is the start of summer and the longest day of the year. For the sun, it’s all downhill for the next six months until the winter solstice.

879 JUNE 23, 2013: Spica: Alpha Star of “Lizard Lady”: The moon is full at week’s start, so expect to see only the brightest stars and planets. Venus has now become the sole guardian of evening twilight. View it about 30 minutes after sundown in the same direction where the “sun goes down” low in the WNW. If you look just a little bit west of south at 10 p.m., you’ll notice two very bright luminaries; the one to the left is Saturn. To Saturn’s right will be Spica, the alpha star of Virgo the Virgin. Although Virgo is one of the zodiacal constellations through which the sun passes, its star pattern is only discernable under the clearest of skies because of the faintness of the other stars which form the constellation. She looks more like a “lizard lady” in a sleeping bag, and I can understand fully why suitors have been reluctant to call. Blue-white Spica, however, is a whole different story. It is truly one of the more luminous stars in the heavens, but because of its 250 light year distance, it appears as only the 16th brightest star of the night. However, if Spica were moved to 10 parsecs or 32.6 light years from our sun, the standard distance to which the brilliance of all stars are compared, Spica would rival the planet Venus in luster. In contrast, Sol would be an inconspicuous star, even from rural locations, similar to the other faint stars which form Virgo, the “Lizard Lady’s” body. Spica is, at the very least, a double star system with two hot, blue-white luminaries orbiting each other in just over four days. Their separation is a mere 11 million miles, about 1/8th the Earth-sun distance. The brighter component of the pair has a surface temperature of 40,800 degrees F, while its fainter companion is just 7000 degrees cooler. The temperature of our sun, in comparison, is only 10,500 degrees F. Catch Spica now; for in the weeks ahead, it will continue to slide towards the western horizon, exiting the deep twilight sky by late August.

880 JUNE 30, 2013: Cause of the Seasons Not Intuitive: The Earth is at aphelion or farthest from the sun on July 5 at 11 a.m., EDT. Just think about that. We will be coming off the Fourth of July celebrations with picnics, fireworks, tank tops, and togs in the fresh blush of summer, and yet we will be at our greatest distance from the sun. One of the big misconceptions that people have about the seasons is that they are a function of Earth-sun distance. Nothing could be further from the truth. They are attributed to the Earth’s axial tilt which “leans” the Northern Hemisphere towards the sun in the summer, so that Sol is higher in the sky and we receive more direct energy from the sun. This also allows the sun to be visible for a longer period of time throughout the day. The net result is an excess of energy, the Northern Hemisphere warms, and everyone goes outside to play. You can demonstrate this effect with a flashlight and a wall. The wall is the Earth’s surface; the flashlight represents the sun. Shine the flashlight straight down onto the wall, and note the area of the wall which the beam illuminates. Keeping the flashlight at the same distance from the wall, allow the light to strike the wall at angles which are less and less steep. You’ll notice the beam expanding into an oval which will elongate itself and cover more and more wall area. It’s the same energy but all spread out. This is what happens in winter when the Earth’s axis “leans back.” Daylight decreases, and the sun appears much lower in the sky. The energy received from the sun lessens and so do the temperatures. All of this leaning towards and away from the sun seems to indicate that the Earth’s axis flips back and forth during a year’s time. This is not the case at all. For now it points towards the North Star keeping the stars of the nighttime sky in step with the calendar. Oh, and how far are we from the sun on Friday, July 5? The number in miles turns out to be 94,508,200.

[June Star Map]

[June Moon Phase Calendar]