Astro Bob: Brighten your week with planet pairings and a rare Jupiter-moon occultation
On May 16 Mars aligns with Gemini's two brightest stars. The following morning a wafer-thin moon covers up the planet Jupiter.
Have you looked at Mars lately? While nowhere near as bright as it was this past winter, it's been strolling across Gemini this month just below Castor and Pollux, the constellation's brightest stars. The trio is easy to find about a third the way up the western sky at nightfall. On Sunday night, May 14, they trace a slightly bent line or arc.
But on Monday and Tuesday nights (May 15-16) they'll make an almost perfectly straight line. Besides being pretty to look at you can also use this opportunity to compare their brightness. Shining at magnitude 1.1, Pollux should appear slightly brighter than both Mars (1.5) and Castor (1.6). Notice that Castor and Mars vary by just 1/10 of a magnitude. Are your eyes keen enough to detect that tiny difference?
Amateur astronomers who observe and estimate the brightness of variable stars achieve this level of discernment after some practice. When I tracked down my first variable through the telescope in 1982 it seemed an impossible standard. Now it's routine. Like most things if you pay attention long enough fine differences become apparent.
We use the magnitude scale to measure the apparent brightness of celestial objects. The original scale, developed by the Greek astronomer Hipparchus, was pretty basic. The brightest stars were called first magnitude and the faintest sixth magnitude. A first magnitude star is about 100 times brighter than a sixth magnitude one.
Astronomers later refined and expanded the scale to account for the fact that Hipparchus's 1st magnitude stars weren't equally bright. How you do describe a star brighter than FIRST magnitude? Go backwards! The very brightest stars were assigned negative magnitudes. For example, Vega is one level brighter than 1st so it's zero (0.0) magnitude. Sirius, the brightest nighttime star, is -1.5 magnitude, Venus -4.6 and the full moon -12.6. The faintest stars visible in the largest telescopes reach magnitude 31 or fainter.
Jupiter meets moon then goes into hiding
Jupiter is just now returning at dawn after its conjunction with the sun on April 11. It appears very low in the east-northeastern sky about 40 minutes before sunrise. An unobstructed view in that direction is essential to spotting it. Because the near-horizon sky is often plagued by haze keep a pair of binoculars handy, too.
On Wednesday, May 17, at dawn, a frail crescent moon will join Jupiter in a close conjunction. From the eastern half of the country they'll be just under 1 degree apart 30 to 45 minutes before sunrise. (Find your sunrise time at timeanddate.com/sun ). Farther west, in Tucson for example, the planet will be pinned to the crescent's bright edge shortly after moonrise. At 4:19 a.m. local time, Tucsonians will see the moon occult (cover up) the planet and its bright moons Europa and Io. Jupiter then pops back into view at the moon's dark edge 52 minutes later in a bright, twilit sky.
Along the West Coast the crescent rises with Jupiter already in hiding. The planet reappears on the moon's opposite side (the dark edge) about 40 minutes before sunrise at 5:17 a.m. PDT. What a glorious sight it will be to watch it emerge and slowly part from the moon like a spring flower opening in sunlight.
Unfortunately, observers in the Midwest and East will see the occultation after sunrise in a blue sky. However, you should still be able to view it — with optical aid — by first blocking the sun with a tree or building and locating the crescent in the southeastern sky.
Binoculars will show Jupiter as a pale, white dot along the crescent's edge. A telescope is best and will offer good views of the moon slowly approaching and then gobbling up the gas giant. Any haze will make it trickier to see, so I hope you'll wake up to blue skies that morning.
Please visit the Jupiter Occultation site for a list of times of Jupiter's disappearance and reappearance for lots of cities across the U.S., Canada and Mexico. Do a search for your town or the nearest larger city. Be aware that the times shown are UT or Universal Time. To convert to Eastern Daylight (EDT) subtract 4 hours; 5 hours for CDT; 6 hours for MDT and 7 hours for PDT.
Jupiter is always accompanied by one or more of more of its four bright satellites. Good news. All four will be on display on May 17. Where it's dark enough observers can watch each Jovian moon disappear one after the another and then reappear on the opposite side of the moon. Bonus! Those same observers may be able to make out the paired shadows of Io and Europa over Jupiter's cloud tops after the event ends. Wow, how often do you get an occultation paired with a double shadow transit? Not often!
Wherever you live you'll get to enjoy some aspect of this auspicious event. Hoping for clear skies (as always)!