The Moon and Planets
After this past weekend’s full moon and lunar eclipse, the moon will wane and rise later each night, while also lingering in the morning daytime sky. Our bright natural satellite will pass through the dim water constellations of Aquarius (the Water-Bearer), Pisces (the Fishes), and Cetus (the Whale), and then end the week on the outskirts of Taurus (the Bull).
If you are out after midnight this week, grab your binoculars or telescope and look along the terminator — the boundary line that separates the lit and dark hemispheres of the moon. All along it, the steeply slanted sunlight will generate spectacular vistas of bright ridges and deep black shadows. It’s the same shadowed terrain we see on evenings around First Quarter, but instead lit from the opposite direction.
The moon will officially reach its Last Quarter phase on Saturday afternoon, August 4. At that time, the 90° angle made by the moon, Earth, and the Sun will cause everyone on Earth to see the moon half illuminated — on its western side. And, an observer standing anywhere on the moon’s near side would see Earth half illuminated, too!
Extremely bright Venus continues to catch our eyes in the western evening sky this week — and it’s not even at its brightest yet! The descending evening ecliptic is now pulling Venus a bit lower each night, but we can observe it until almost 10:30 pm local time. In a small telescope, the planet’s disk will resemble a first quarter moon, lit on the sunward side (although your telescope might flip the view).
Our opportunities to enjoy Jupiter will soon be over for this year. This week, the bright planet will continue to shine very brightly in the southwestern sky after dusk, and then set in the west-southwest at just before midnight local time. The bright star sitting just to the left of Jupiter is Zubenelgenubi, the brightest star in Libra (the Scales). In binoculars, you’ll plainly see that it is a pair of stars. While you have the binoculars handy, see if you can see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) flanking the planet.
On Tuesday, August 31, Io’s shadow will begin a crossing (transit) of Jupiter’s disk at 9:49 pm that ends at midnight. A reasonable backyard telescope will show the black shadows, but a very good telescope is needed to see the moons themselves. More shadow transits are available in other time zones around the world, including some double shadow ones.
The Great Red Spot (or GRS, for short) takes about three hours to cross Jupiter’s disk. But the planet’s 10-hour rotation period (i.e., its day) means that the spot is only observable from Earth every 2–3 nights. If you’d like to see the GRS, use a medium-sized telescope (or larger). You’ll have your best luck on evenings with steady air — when the stars are not twinkling too much. Try to look within an hour before or after the following times: Tuesday, August 21 at 9:02 pm and Thursday, August 2 at 10:41 pm. All times are given in Eastern Daylight Time (EDT), so adjust for your local time zone.
This week, medium-bright and yellowish-tinted Saturn will be visible from dusk, when it’s shining over the southern horizon, until it sets in the west at about 3:30 am local time. It will be the first star-like object to pop out of the southern twilight sky. The ringed planet is spending this summer just east (to the left of) the Milky Way, and just above the Teapot-shaped stars that form Sagittarius (the Archer). As the sky darkens, even a small telescope should be able to show you some of Saturn’s larger moons, especially Titan. Because Saturn’s axis of rotation is inclined about 27° from vertical (a bit more than Earth’s is), we can see the top surface of its rings, and its moons can appear above, below, or to either side of the planet. The tilted axis also gives Saturn seasons. But they last 7.5 years, as opposed to our “short” 3-month-long seasons.
At visual magnitude 5.8, blue-green colored Uranus is visible between midnight and dawn without optical aid under very dark skies, or in binoculars and telescopes under moderately light-polluted skies. The ice giant planet is located in the eastern sky, about five finger widths to the left of the modestly bright star Torcular (Omega Piscium), which is above the “V” where the two starry cords of Pisces (the Fishes) meet. The major asteroid Juno is in the same region of the sky, about a fist’s diameter to the lower left of Uranus.
Distant, blue Neptune, among the dim stars of Aquarius (the Water-bearer), will be observable in telescopes in the eastern sky after it rises after 10 pm local time. This week, look for the magnitude the 7.8 planet sitting 1.5 finger widths to the right of the naked eye star Phi (φ) Aquarii and about 4 finger widths to the left of the bright star Hydor (Lambda Aquarii).
I’ll post sky charts for the observable planets here.
Mars at Closest Approach
Last Friday, Earth passed Mars on the “inside track”, a term astronomers call opposition. On that day, Mars rose in the east as the sun set in the west. And because we were so much closer to Mars, it appeared brighter and looked larger in binoculars or a telescope.
But our minimum distance from Mars will occur this Tuesday, July 31. At that time, the Red Planet will be 57.6 million km or 0.385 astronomical units (the mean sun-Earth separation) from Earth. That translates to only 3 minutes and 18 seconds for radio signals to reach the planet, or vice versa. It will continue to look spectacular, so keep your telescope handy!
Here’s how to find and see Mars this week. Tonight (Sunday) Mars will rise at about 9 pm local time (depending on your latitude). It will climb until 1:15 am local time, when it will reach an elevation of about 20° (or two outstretched fist diameters) above the southern horizon. This is the best time to view the planet in a telescope because it will then be shining through the least amount of Earth’s distorting atmosphere. Note that 20° is lower than many trees and buildings, so a clear southern vista is essential.
Because Mars is still close to opposition, it will descend into the west in the wee hours and set around dawn. Mars will be rising about 5 minutes earlier every night, so on Tuesday it will rise just before 9 pm.
If we have clear skies, the planet will be impossible to miss. It will be brighter than anything nearby, except the late-rising moon. The planet’s red coloration will be obvious — as opposed to Saturn’s mere tint of yellow. The planet will not rapidly move location, or flash or blink. Anything doing that is a plane — keep hunting.
The farther south you live, the higher Mars will climb. An observer in Florida will see Mars nearly halfway up the sky after midnight. And someone at the latitude of Sydney, Australia will see Mars directly overhead at midnight!
Wherever you are, even with a small telescope on a night of good seeing (i.e., with clear, steady air), you should be able to see the Mars’ southern polar cap of frozen CO2 and water ice, and also some darker and lighter regions on the planet. Because the Mars’ rotational period (its day) is about 38 minutes longer than Earth’s, by viewing the planet over many nights you can see different parts of its surface. In fact, it would take you 41 nights of observing to see the entire globe of Mars. But since we have about 7 hours of darkness during nights in late July, you could also observe Mars from about 10 pm local time, soon after it rises, until 4:30 am just before it sets, and see about one-quarter of the globe in a single night.
A global dust storm has recently enveloped the planet — hiding its surface. But skilled planetary imagers like Damian Peach have reported that the storm is abating. Fingers crossed!
Don’t worry if your Mars viewing is clouded out this week. After Tuesday, Earth will slowly begin to pull away from Mars, but Mars will decrease in apparent size more slowly than it has been increasing. In fact, it will look 90% as large for the next month! Its visual brightness will also remain intense for weeks to come.
Astronomy Skylights for this week (from July 29th, 2018) by Chris Vaughan.
Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.
