April 27, 2016 — Jupiter, Leo Stalks Jupiter, and Mercury Transits Sun Soon!
The Moon and Planets!
The next few evenings are still decent to catch Mercury low in the western sky after sunset as the planet drops towards the Sun. Find a location with a low horizon (or perhaps a west-facing balcony) and, after the Sun is completely down, about 8:30 pm, look for the planet no more than 12° (a closed fist diameter) above the horizon. After 9 pm it will be sinking into the haze. Mercury is a reasonably bright dot above, and bit to the south of, where the Sun went down. Binoculars may help, but be sure that the Sun has completely set first! If you use a telescope on Mercury, look for the planet’s waxing crescent phase. Today, only 20% of its disk, the sunward side, is illuminated.
Mercury is gradually moving between us and the Sun. On Monday, May 9, from 7:15 am to 2:40 pm Eastern Time, the little planet’s orbit will carry it across the Sun’s disk. This special Transit of Mercury event will be visible over large parts of the Earth. The transit can ONLY be observed safely with special filters. Your local astronomers will likely be setting up special solar telescopes for public viewing.
The king of the planets, Jupiter, remains the dominant object high in the southern sky every evening, sitting below the stars of Leo the Lion. As the sky darkens around 9 pm, Jupiter is well above the southeastern horizon. By 10 pm, it is high and due south, and it sets in the west around 4:30 am. Some particularly good times to observe Jupiter are listed below. Jupiter is already in the west half of the sky when twilight ends, so the Jupiter events are tapering off for the year.
Slowly but surely, Mars and Saturn are moving into visibility at a comfortable hour of the evening. By the end of this week, bright reddish Mars will be leading dimmer, yellowish Saturn across the sky from 10:30 pm to dawn. The two planets are about a palm’s width apart. Around 3 am, the apparent motion of the sky has carried them due south. The bright star they hover over is the red supergiant star Antares, the brightest star in Scorpius the Scorpion. As the dawn sky lightens, the two planets are in the low southwest sky. Mars is steadily brightening as we prepare to “pass it on the inside track” in mid-May.
Let’s Look at Leo the Lion
For millennia, sky-watchers have imagined the stars in the night sky linking into patterns — forming humans and animals and inanimate objects. We call these groupings constellations, after the latin for “stars grouping together”. Each culture has assigned their own spin on the heavens, usually naming the patterns after things in their everyday experience. For example, south sea islanders saw outrigger canoes, and the Inuit saw Ursa Major’s Big Dipper as the Caribou and Cassiopeia’s “W” as the Blubber Container and adjacent Lamp Stand.
In modern-day astronomy, the entire sky is divided into 88 officially recognized constellations. The manner in which the stars are connected into stick figures is not regulated, but the boundary lines dividing the constellations is. That way, there are no gaps in the coverage, and every object in the heavens can be placed within one of the 88 regions. By the way, astronomy has a long tradition in China. Chinese astronomers combined fewer stars at a time and defined several hundred Chinese asterisms or 星官, xīngguān.
A handful of constellations are so obvious that many independent cultures assigned the same figure to those stars. A perfect example of this is the spring constellation of Leo the Lion. The lion was identified as early as 1,000 BCE by the Babylonians, and later by the Egyptians, Greeks, and Romans. To the Greeks it represented the Nemean lion slain by Hercules during his labours. Only the beast’s own claws were sharp enough to slice its hide, and he used them against it.
It’s easy to find and recognize this constellation, even from suburban skies. Head out on the next clear evening and face the southeast. Leo is a large constellation situated more than halfway up the evening sky, and just above the imaginary Ecliptic line which the Sun, Moon, and planets all move along. For this reason, Leo is one of the twelve Zodiac constellations. He is tilted upwards with his head facing west and his body sloping down to the east (left). This spring, Jupiter is sitting about two fist widths to the upper right of the lion’s brightest star, Regulus. For this year only, Jupiter sits right below the lion’s torso.
Regulus means “Little King” in Latin and its Arabic name Qalb Al Asad translates to “the heart of the Lion”. A blue-white star with a visible small companion star nearby, it’s the 21st brightest star in the heavens, and it sits almost on the Ecliptic, making it a star commonly occulted by the Moon and the inner planets.
From Regulus, move upwards and trace out another five modest stars forming a backwards question mark, or Sickle. This represents the lion’s neck, head, and mane. The second star up from Regulus is another reasonably bright star named Algieba, or “the forehead”. In a backyard telescope, Algieba splits into a very pretty matched pair of yellow stars.
From Algieba, cast your gaze a bit more than fist’s width to the lower left to find a star of similar brightness named Zosma, situated at the lion’s hip. Then, angling down and to the left another fist’s width, we find a star at the lion’s tail, Denebola, “tail of the lion”, the second brightest star in the constellation. Denebola is a young star only a few hundred million years old. It emits quite a bit of infrared radiation, suggesting that this young sun may have a planet-forming dust disk around it.
The last major star of Leo, named Chertan, sits to the right of Zosma and Denebola. The three stars form a nice triangle. This is where things get interesting! Leo is a favorite of amateur astronomers because it is located well away from the plane of the Milky Way, in a direction of sky that contains a nice selection of relatively nearby and bright galaxies.
Draw a line from Zosma to Chertan and, about two finger widths farther from Chertan, is a famous trio of spiral galaxies called the Leo Triplet. All three galaxies (two of which are on Messier’s list as M65 and M66), when viewed from a dark sky location, can be framed in a good telescope at low magnification. I’ll post a photo on tumblr here. Another sprinkling of galaxies is located midway between Chertan and Regulus, two finger widths below the line joining the two stars. This group contains M95, M96, and M105 — plus another non-messier galaxy. And that’s just the beginning…
For the next couple of months, Leo will slowly stalk westward across the heavens. Take a few minutes to look at this majestic and ancient constellation.
Special Events for Jupiter
You can use even a modest telescope to observe interesting events among Jupiter’s four bright Galilean Satellites named Io, Europa, Callisto, and Ganymede (in order of distance from Jupiter). Below are the best Jupiter moon events this week (all times in EDT). (Southern hemisphere observers should swap left and right in the notes below.)
On Fri, Apr 29 at 12:30 am Io passes behind Jupiter’s right edge, and then pops out of Jupiter’s shadow on the opposite side at 3:49 am. On Friday evening from 9:41 to 11:56 pm, Io will transit, followed by its shadow from 10:44 pm to 12:59 am. On Sat, Apr 30 from 12:37 to 3:24 am, Europa transits, followed by its shadow from 2:46 until the planet sets after 4 am. On Saturday evening at 10:18 pm, Io pops out of Jupiter’s shadow (on the left side). On Sun, May 1 at 11:53 pm, Europa will pop out of Jupiter’s shadow (to the left of the planet).Get out your scope, big or small, and try them!
The Great Red Spot (GRS) can also be observed in a decent backyard telescope, if you know when to look. Great Red Spot transits take about three hours to complete. Here are the eastern daylight times of the GRS transits visible in the GTA this week, given for the midway point (i.e., 90 minutes into it): Mon, Apr 25 at 12:02 am, Mon, Apr 25 at 7:53 pm as twilight ends, Wed, Apr 27 at 1:40 am, Wed, Apr 27 at 9:32 pm, Fri, Apr 29 at 3:19 am, and Fri, Apr 29 at 11:10 pm.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.
Stargazing News for this week (from April 27) by Chris Vaughan. (Feel free to pass this along to friends and send me your comments, questions, and suggested topics.) I post these with photos at http://astrogeoguy.tumblr.com/ where the old editions are archived. You can also follow me on Twitter as @astrogeoguy! All times mentioned are in Eastern Daylight Time. Please click this MailChimp link to subscribe to these emails. If you are a teacher interested in a guided field trip to the David Dunlap Observatory, details are available at www.astrogeo.ca.