June opens with an impressive evening twilight lineup of three planets—all outshining June’s brightest stars, and all gathered within one constellation, Gemini, the Twins.
Look between the west and west-northwest; the best viewing may occur about one hour after sunset. On June 1, the lineup spans 23°. Venus is its brightest member, with second-brightest Jupiter, within 8° to its upper left, and Mercury, within 16° to Venus’ lower right. Gemini’s namesake “Twin” stars—Pollux, with dimmer Castor 4.5° to its right—appear to the upper right of Jupiter and above Venus. If there are obstructions that might block your line-of-sight toward Mercury low above the west-northwest horizon, start your viewing earlier, and use binoculars.
Historically, the characteristic that defined a planet apart from a star, is that planets were observed to change their positions against the background of fixed stars. So, in addition to the five anciently-known sky objects we still regard as planets—we’ll meet Saturn and Mars in the morning sky—the sun and moon were counted as planets, too! The sun appears to make one circuit around the belt of zodiac constellations in one year, while the moon does so in 27.3 days. (It takes an extra 2.2 days for the moon to lap the sun, completing the cycle of phases, from full moon to the next full moon for example, in 29.5 days.) Mercury and Venus oscillate around the sun’s position in the zodiac, never wandering more than 28° or 47° from the sun, respectively. Mars, Jupiter and Saturn have no such limit, and can appear anywhere from 0° to 180° from the sun. The average time required for one trip around the zodiac is about 1.9 years for Mars, 11.9 years for Jupiter, and 29.5 years for Saturn.
This month, we’ll witness some striking arrangements of the moon, planets and stars, and several are described below. You can find bright markers within the belt of the zodiac by extending the line Mercury-Venus-Jupiter, beyond Jupiter to the following first-magnitude stars: Regulus, heart of Leo, the Lion, well up in the west-southwest, 36° to the upper left of Jupiter in early June; Spica, the spike of grain in the hand of Virgo, in the south-southeast to south, 54° east of Regulus; and the red supergiant star Antares, heart of Scorpius, the Scorpion, low in the southeast, 46° east of Spica.
The most spectacular event this month is the conjunction of the two brightest planets, Venus and Jupiter, 1.6° apart on the evening of Tuesday, June 9. That evening, Mercury appears 13° to the pair’s lower right. The planets appear close together, but are really far apart. Their distances from Earth in light travel time are: Mercury 7.7 minutes; Venus 9.9 minutes; and Jupiter 50.3 minutes—so Jupiter is more than five times more distant than Venus! The planets are moving east against background stars—Venus by nearly 1.2° daily; and Jupiter by 0.2° daily. Venus-Jupiter appear within 5° for 10 evenings, June 4-13, easily fitting within the field of view of binoculars, even those magnifying as much as 10X. Watch for daily changes! Mercury almost matches Venus’ eastward motion for a few days, so it lingers 13° to the lower right of the brilliant planet during June 8-14, before slowing and widening the gap.
Also note the patterns the planets form with stars, especially Pollux and Castor. In the case of fast-moving Venus, you can even note the change from one night to the next. On May 30, Venus was below the Twins, forming an isosceles triangle, 10° from each. On June 7, Venus passes 4.7° to the lower left of Pollux, while Jupiter stands only 2.3° to the left of Venus.
On June 10, a line from Castor to Pollux—4.5° apart and extended 6° to the left—points directly to Venus, which is accompanied by Jupiter 2° below. Mercury entered Gemini on June 1, and from then through June 10, the three evening planets remain within the boundaries of Gemini. But on June 11, Venus crosses into Cancer, where the Beehive Cluster resides. Venus will graze the northern edge of the cluster on June 19. Use binoculars to observe the cluster’s stars of magnitude 6 and fainter for a few nights before and after. Jupiter crosses into Cancer on June 21—on the same date that summer begins at 1:25 a.m. PDT. Also that day, the moon reaches first quarter phase, half-full and 90 degrees east of the sun, which means that the moon marks approximately where the sun will appear in Virgo, at the beginning of autumn. On June 28, Venus passes from Cancer into Leo, where it will pass closely north of the star Regulus on July 9.
Mercury remains to the lower right of the two brilliant planets throughout its evening apparition. During June’s first week, it outshines June’s brightest stars, all of zero magnitude: golden-orange Arcturus, high in the southern sky; blue-white Vega, well up in the east-northeast; and yellowish Capella, low in the northwest. Mercury attains its highest altitude during June’s second week, and fades to magnitude +0.3 by week’s end.
On June 10, Mercury, below the Twin stars Pollux and Castor, forms an isosceles triangle with them, 11° from each. Mercury is within 7 degrees to the lower left of Pollux (magnitude +1.1) June 15-21, and closest, at magnitude +0.7 and 6.5 degrees away, on June 18. On June 24, Jupiter lies on a straight line extended left from Castor through Pollux, while Mercury approaches within 3 3/4° to the lower right of Jupiter. But Mercury has faded to magnitude +1.4, making it quite a difficult target low in bright twilight. Use binoculars!
The moon has some noteworthy gatherings. Let’s start in the morning sky, where the waning moon can be followed through June 13 or 14; except for June 14, a good time for viewing might be 1 to 1 1/4 hours before sunrise. On June 8, the moon reaches last quarter phase, half-full and 90° west of the sun. Note the two first-magnitude “stars” (actually planets) in the eastern sky, to the lower left of the moon. The higher one, in the east-southeast 24° from the moon, is Saturn. A telescope will show its rings, now tipped nearly 9° from edgewise. The other one, reddish Mars, is low in the east-northeast to east, 32° to the lower left.
In following days, watch the waning crescent moon slide down the line of morning planets, passing a few degrees above them: within 6° to Saturn’s upper left on June 10, and within 6° above Mars on June 12. On June 13, try to observe from a place with an unobstructed view toward the east-northeast, so you can observe the 4 percent old crescent moon rising 12° to the lower left of Mars. Go out early, and use binoculars or a telescope, starting your viewing about 1 1/2 hours before sunrise, so you can observe the moon in a dark sky, closely surrounded by members of the Pleiades star cluster.
Spotting the moon on the morning of June 14 will be very challenging. If you want to try, you’ll need a very unobstructed view about 30°-32° north of east; very clear skies; and optical aid. You’ll have no choice but to look during bright twilight, 30 to 35 minutes before sunrise. If you spot the 1 percent very old crescent, note the times you first and last observe it, the instrument you used, and sky conditions. The new moon occurs later that day, at 7:54 p.m.
In the evenings, the young moon in twilight on June 15 will be quite easy to see, even for the unaided eye, given good sky conditions and an unobstructed view toward the horizon about 30° north of west. Starting from Venus, go 6.3° to the lower right to Jupiter, and next, go 7.5° to the lower right of Jupiter to find Mercury. From Mercury, go about 11° to the lower right to the illuminated lower edge of the lunar crescent. The entire span of four solar system bodies, from Venus to the lunar crescent, is 24°. The moon will be 4° above the horizon, while the sun will be 14.4° to the moon’s lower right and 9° below the horizon. The moon’s age will then be about 24 hours and 50 minutes. If you want to set a personal record for the youngest moon you’ve ever seen, or shortest time between opposing crescents, start your session earlier; use optical aid; and keep a record of your observations, noting times, instruments used and sky conditions. If you spot the moon on both June 14 at dawn and June 15 at dusk, then you will have captured opposing crescents on consecutive days, a feat few folks have accomplished.
What makes this occasion favorable is that this new moon is near perigee, and it passes north of the sun, so the moon moves more quickly through the zone of invisibility. Another factor in favor of observing the moon on both June 14 and 15 is that June days are long, giving a larger time interval between dawn on June 14 and dusk on June 15. Viewers in eastern U.S. have an easier chance at the morning crescent than we do, because it’s farther ahead of the new moon and farther from the sun.
On the evenings following June 15, for better views, shift your viewing time into the darker skies of one to 1 1/4 hours after sunset. On June 16, from Venus (at the upper left) to Mercury (at the lower right), the span of four solar system bodies is only 14°: Jupiter 7.2° is to lower right of Venus; a 6 percent crescent moon is 3.5° to the lower right of Jupiter; and Mercury is 4.2° to the lower right of the lunar crescent.
On June 17, start with the 13 percent crescent moon in the west. Find Venus, Jupiter and Mercury within 4°, 12° and 18° to moon’s lower right, respectively. Earlier in the day, the moon occults, or covers Venus. The event will be visible well up in the eastern sky in the daytime, with binoculars or a telescope. From Reno, Venus disappears behind the invisible leading dark edge of the moon at 11:36 a.m. and reappears along the moon’s sunlit edge at 12:50 p.m.
After the moon passes Venus, watch it slide past the bright zodiacal stars mentioned earlier: Regulus in Leo on June 18 and 19; Spica in Virgo on June 22 and 23; and Antares in Scorpius on June 26 and 27. Full moon occurs on June 29.
On the morning of June 29, as the summer triangle of Vega, Deneb and Altair floats high in the west, a long line of three solar system bodies is on fine display: the full moon, low in the southwest; Saturn, well up in the east-southeast to southeast; and Mars, low in the east-northeast to east. Aim binoculars at Mars, and enjoy an improved view of the Pleiades cluster about 4° to the red planet’s upper left. Aldebaran, red-orange eye of Taurus, the Bull, is 11° to the lower left of Mars. Capella, the Mother Goat star and northernmost of the stars of first magnitude or brighter, is low in the northeast, while Fomalhaut, Mouth of the Southern Fish, is approaching due south.
Many of the sky events described above are illustrated on the Abrams Planetarium Sky Calendar. For $12 per year, subscribers receive quarterly mailings, each containing three months of calendars and evening star maps. Subscription info and a free printable sample are available at abramsplanetarium.natsci.msu.edu/sky-calendar/index.aspx.
Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys promoting sky watching to folks of all ages in and around Palm Springs and East Lansing, Mich. Robert Miller, who provided the twilight charts, 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.
