Click here for Dave's preview of sky events in 2016.
For: June 20-26
At 5:34pm today we reach the summer solstice. This is a time when the Sun is directly above the Tropic of Cancer at latitude 23.5 degrees north. At our 40 degree latitude, we see the Sun 73.5 degrees high and shadows are short near noon. We’ll also experience roughly 15 hours of daylight – notice the time when it gets dark! “Solstice” means “Sun still.” If you measure the shadow of a pole at the same time each day, the length won’t change much, hence the name. It won’t be too long, though, before the shadows slowly lengthen as our noon Sun gradually lowers. With the Sun so high, its light is most concentrated on the ground resulting in warmer temperatures.
For: June 27-July 3
The brightness of sky objects is measured by an arbitrary system of “magnitudes” that date to the second century BC. Hipparchus had six brightness categories with “first magnitude” being the brightest. That system has been refined a bit but it’s basically still in use. Think of a number line. Our standard candle is Vega (the bright star in the east) at “zero.” The brightest night time star is Sirius at -1.5. The full Moon is about -12. On May 30, Mars was at magnitude -1.78 but, the planet will dim as the distance between Earth and Mars increases. In the next two weeks Mars will fade to -1.1. Compare it to the star Antares (to the lower left of Mars) which is magnitude +0.96.
For: July 4-10
Today the Juno spacecraft officially reaches Jupiter! Launched nearly five years ago, the craft flew beyond the orbit of Mars, then around the Earth again to gain the energy to reach Jupiter. Juno will orbit Jupiter 37 times in a polar orbit in order to study the planet’s composition, magnetic field, and gravity. Jupiter is one of the keys to the solar system as we think it may have formed before the other planets. Photography isn’t a primary goal of Juno but there is a “JunoCAM” on the craft, so stay tuned for some wonderful photos. Look for the Moon in the west Thursday evening then near Jupiter on Friday. There is an open house at the CUAS Observatory Saturday (cuas.org).
For: July 11-17
Pluto is well-placed in the sky this week, situated above and left of the Teapot of Sagittarius in the south. That’s the good news; the bad news is that it’s faint! At magnitude 14, you probably need a telescope with a ten or twelve inch mirror to have a chance at seeing it. Then you need some very good charts, too. Pluto was closest to our Earth late last week. At a distance of nearly three billion miles, light still takes over four hours to get out to Pluto. Those attending the CU Astronomical Society meeting Thursday night at the planetarium (7pm) will be entertained by a travelogue conducted by Dick & Ellen Robrock. Join us!
For: July 18-24
I have a challenge for you this week. Using binoculars, when can you first see Venus? Venus passed behind the Sun in early June and now slowly moves out from behind its brilliance. Venus sets just after the Sun, so you will need an unobstructed horizon. Look in the west-northwest just after the Sun sets. Also this week, Mercury will appear above and to the left of Venus, though Mercury will appear much fainter. Venus and its highly reflective clouds will be easier to see each evening, but its rise out of the twilight is gradual. It won’t set in a dark sky until the fall. Give it a shot and let us know at email@example.com when you’ve seen it!
For: July 25-31
One could say “meteor season” begins this week. Though there is no official “season,” you’ll start to see a few more “shooting stars” in the sky. Meteors are bits of dust in space, roughly the size of a pencil eraser. When they slow down in the atmosphere, their energy is transferred to the air, making it glow. When the Earth encounters a stream of dust, usually left by a comet, we have a meteor shower. The Delta Aquarid and the Alpha Capricornid showers are expected to peak this week. Though the meteors enter the atmosphere at roughly parallel trajectories, to us they appear to emanated from a spot in the sky called the “radiant.” This is how the showers inherit their names.
See Dave Leake's "Prairie Skies" column in the Champaign-Urbana News-Gazette each Monday morning.