By Mike P. Usher
Every now and then instead of facing a particular point of compass, it’s fun to look straight up — technically called the zenith. Looking up at the zenith has a couple of advantages over other directions: the sky becomes darker, and there is less atmosphere to absorb light so the stars tend to be brighter. The major difficulty is the human body is not well designed to spend an extended period of time staring straight up. Soon your neck starts hurting, and if you move suddenly, you could get dizzy and trip over something. It’s even worse if you use binoculars. Fortunately, a simple solution is close at hand; reclining chairs used around pools are the perfect stargazing accessory here.
Tonight, too-bright-to-miss Jupiter dominates the zenith and indeed the entire sky. It’s the only planet in the sky at the moment although Mars pops up in the East about 10 PM with Saturn following shortly after midnight. Now is the perfect time to whip out your trusty binoculars, and see Jupiter’s four bright satellites. Ranging from just slightly smaller than our own Moon to larger than the planet Mercury, the Galilean satellites are easy to spot with just a small pair of firmly-held binoculars. Indeed, all would be bright enough to see with the naked eye were it not for the overwhelming glare of Jupiter. Occasionally, there are reports of very rare people who can spot one (probably Ganymede) without optical aid.
Jupiter is within the borders of Gemini, the twins where the matching pair of Castor and Pollux form the heads of the stick-figure twins. Pollux is just a bit more luminous than Castor, but reports from ancient times indicate that Castor was the brighter star. It is uncertain if there was a mistake made or if there really has been a change. Several other changes in the sky might have occurred in the last few millennia. For example, the nearby (but not on tonight’s chart) Pleiades are almost universally known as the “seven sisters,” but only six stars are plainly visible to the unaided eye.
Now might be a good time to try and pick out the ridiculously faint constellation of Cancer, the crab. The brightest star in this constellation is only magnitude 3.5, while the Praesepe star cluster within it reaches 3.1. Without a doubt no constellation would have been placed here in ancient times were it not for the fact Cancer occupies a position along the ecliptic — the apparent path the Sun takes during the course of a year. The ecliptic was important to the ancients for several reasons; they must have believed every spot along it deserves it’s own constellation if for no other reason than to provide a convenient marker.
See you next time!
Mr. Usher is a Director of the Everglades Astronomical Society, which meets the second Tuesday each month at 7 PM in the Norris Center, Cambier Park, Naples.