July 2002 Sky from the Keeble Observatory
How many planets? To the ancients, there were seven - including Sun and Moon. These were the lights that wandered against the background stars, drifting more or less steadily to the east. The Sun takes about 4 minutes longer in its daily ("diurnal") motion to circle the sky than do the stars. Our Moon takes about 90 minutes more. Mercury and Venus play a celestial game of tag with the Sun, sometimes to the east (as an "evening star") and sometimes to the west ("morning star"). Mars, Jupiter, and Saturn circle the zodiac at a more sedate pace, though sometimes confounding by appearing to stop and turn to the west before resuming the general eastward trend.
With the advent of telescopes, Uranus was discovered in the 18th Century. Apparent discrepancies in its orbit led to the prediction and subsequent discovery of Neptune. A similar effort led to the discovery of Pluto - though errors in the calculations had them looking at its location by mistake! Just as well - it's too small to have caused the anomalies, even if it were in the right place. And, we have now refined the measured orbits, to the point that they are well determined without invoking either Pluto or any purported "tenth planet." The anomalies are resolved, there's no "need" for a tenth planet orbiting the Sun. Indeed, some are now suggesting that Pluto should be demoted to a "trans-Neptunian object" rather than calling it a full-fledged planet.
But, planet hunters are on the trail of more exciting stuff: planets orbiting other stars! And, they're finding lots of them. The latest count is over 90 "extra-solar" planets discovered to date. Planets do not give off their own light, rather they reflect light from the stars. In particular, we see the other planets in our solar system by reflected sunlight. Extra-solar planets would be brightest in reflected light from the stars they orbit. And that's a problem. Stars are so much brighter than the planets that it is virtually impossible with present equipment to separate the starlight from the planets' light. Maybe in a year or so, with technology still in the design stage.
The most commonly applied technique at present is to look for the gravitational tug of the orbiting planet on the star itself. For orbital motions along the line of sight, there will be a periodic "Doppler shift" in the wavelengths of spectral features from the primary star. (Much the same way that radar can be used to determine the speed of a car.) These can be used to determine approximately the mass of the planet and its orbital parameters.
Most of the planetary systems thus discovered bear little resemblance to our solar system. The current zoo of extra-solar planets is populated by giant planets several times the mass of our own, more familiar Jupiter. Many are in orbits which are highly eccentric ellipses, unlike our own solar system's nearly circular orbits. Several orbit closer to their primaries than Mercury does to our Sun. Should we be worried? Not yet … the technique is biased, in the sense that it is easier to detect the extreme cases. We should not be surprised that we are finding those extremes!
The good news is that researchers have recently announced discovery of yet another extra-solar planet, this one orbiting the relatively nearby, Sun-like star 55 Cancri. The latest denizen in the zoo is only a 1-3 times more massive than Jupiter, and its orbit is nearly circular at a radius of 5 astronomical units - almost exactly where Jupiter orbits our Sun.
Lunar phases for 2002 July: Last Quarter, 1:19 pm on the 2nd; New Moon at 6:26 am on the 10th; First Quarter on the17th, at 12:47 am; Full Moon on the 24th at 5:07 am.
Venus continues its bright stay in the early evening skies. Look for it as the first bright "star" to emerge from twilight after the sunset. Below and to the right, you may catch Jupiter and Mars together within one degree on the first through third. Jupiter will be gone behind the Sun by month's end, reemerging into the morning sky next month. In the predawn twilight, you can still find Saturn rising about an hour before sunrise at the beginning of the month, three hours earlier by the end of the month. It's worth getting up (or staying up!) to see this. The ring plane is tilted almost 27 degrees to the line of sight, making the view with a small telescope or binoculars quite spectacular. Mercury is still visible in the morning as well, but it will disappear into the solar glare by mid-month.
If you're keeping track, Earth is at its greatest distance from the Sun (aphelion) on the 6th. That should be the coolest part of the year, but our seasons are primarily driven by the tilt of our axis, and the Sun remains above the horizon in the northern hemisphere for longer at this time of year than in our winter. Southern hemisphere seasons are somewhat moderated by the greater percentage of oceans versus land mass.
Our overhead view at mid-month, approximately 9:00 pm, finds the faint circlet of the constellation Corona Borealis almost at zenith. Your eye will more likely be drawn to the bright star Arcturus, a bit to the southwest of zenith in the constellation Bootes. Lower to the southwest is Spica, the brightest star in Virgo. Turn to the south, and somewhat closer to the horizon you will find the bright red Antares marking the constellation Scorpio. Regulus, the heart of Leo the Lion is to the west, just below the more brightly glowing Venus. Vega marks the eastern sky, half-way from horizon to overhead. Sweeping your binoculars from Vega to Arcturus, look for the wispy globular cluster in Hercules. Nearer Vega, on a clear moonless night (if you can find one in Virginia in July!) you may pick out the smoke-ring shape of the Ring Nebula.
If you're interested in more detailed descriptions of the night sky on a month-to-month basis, check out Skymaps.com - you can download and print your own map, with descriptions of what to look for and where to look.