The Moon and Planets

This week, the moon remains prominent in the evening sky worldwide, waxing towards its full moon phase on Friday. On Tuesday night, the moon will land about two fingers to the upper right of yellowish Saturn. And on Friday, the Full “Buck Moon”, “Thunder Moon”, or “Hay Moon” will grace the sky to the upper left of Mars.

This full moon is occurring only 13 hours after apogee, the farthest point from Earth in the moon’s orbit. That will give us the smallest apparent full moon size of 2018. Meanwhile, a total lunar eclipse will be visible from Africa, the Middle East, India, and western Australia. Eastern Australia and Southeast Asia will see a portion of the eclipse before the moon sets and morning twilight arrives, while in Europe and Eastern South America the eclipse will already be in progress when the moon rises. North America will not see any of this eclipse. Maximum eclipse occurs just east of Madagascar at 20:22 UT. The moon will cross just north of the center of the Earth’s darker umbral shadow, setting up conditions for an extra long and very dark eclipsed moon. At greatest eclipse, the moon will be sitting a palm’s width north of Mars — that’s two reddish objects for the price of one!

(Above: Venus gleams in the western sky after sunset as shown here for 9:15 pm local time on July 22, 2018. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

Brilliant Venus continues to brighten in the western evening sky this week — easily visible until it sets at 10:45 pm local time. The descending evening ecliptic is now pulling Venus a bit lower each night. In a small telescope, the planet’s disk will exhibit a waning gibbous (60% illuminated) phase, lit on the sunward side (although your telescope might flip the view).

This week, Jupiter continues to be the object shining very brightly in the southwestern sky after dusk. It will set in the west-southwest at about 1 am 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 them out, can you also see Jupiter’s four Galilean moons (Io, Europa, Callisto, and Ganymede) flanking the planet? Astronomers recently announced the discovery of another 12 moons around the giant vacuum-cleaner of a planet — bringing the total to 79! Many of the smaller moons are former asteroids captured by Jupiter’s strong gravity.

(Above: All month long, Jupiter will sit to the upper right of the easy-to-see double star Zubenelgenubi in Libra, as shown here for 10 pm local time on July 22 2018. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

On Sunday, July 22, Europa’s shadow will begin a crossing (transit) of Jupiter’s disk at 10:46 pm that ends when Jupiter is setting at about 1 am. On Tuesday, July 24, Io’s shadow will complete a transit at 10:04 pm that began earlier in twilight. 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.

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: Thursday, July 26 at 9:52 pm and Saturday, July 28 at 11:31 pm. All times are given in Eastern Daylight Time (EDT), so adjust for your local time zone.

(Above: A trio of planet images captured and processed by award-winning RASC member Rick Foster, taken from his home in Markham, Ontario.)

This week medium-bright and yellowish Saturn will be visible from dusk, when it’s shining over the southern horizon, until it sets in the west at 4 am local time. 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). Once it’s dark, a small telescope should be able to show you some of the ringed planet’s larger moons, especially Titan. Because Saturn’s axis of rotation is tipped 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.

Blue-green coloured Uranus is visible in binoculars and telescopes between midnight and dawn. The ice giant planet is located in the eastern sky, about four 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 about a palm’s width below Uranus, in the same region of sky.

Distant blue Neptune, among the modestly-bright stars of Aquarius (the Water-bearer), is observable in telescopes in the eastern sky after it rises after 10:30 pm local time. This week, look for the magnitude 7.9 planet sitting one finger width to the right of the naked eye star Phi (φ) Aquarii and about 4.5 finger widths to the left of brighter Hydor (Lambda Aquarii).

Mars at Opposition and Closest Approach

Well, this is it. It’s finally here! The week when Earth’s orbit will bring us closer to Mars than we’ve been in 15 years. This is a worldwide event, although people viewing Mars from low latitudes and from the Southern Hemisphere will see the Red Planet much more clearly than the folks in mid-northern latitudes. Let’s break it all down.

(Above: The orbits of Mars and Earth as viewed from high above the northern pole of the sun. The radial lines are drawn through the two planets at opposition dates from March, 2012 through January, 2015. The current opposition is among the closest the two planets ever get.)

Mars has an orbit that is about 50% farther from the sun than Earth’s, on average. That causes Earth to complete an orbit of the sun faster than Mars does. In fact, Earth circles the sun almost two times in the time Mars takes to complete one orbit. If the planets were placed at the same starting line on an imaginary racetrack around the sun (as they will be this Friday), when Earth returned after lap one, slowpoke Mars would be out of sight on the opposite side of the sun. When Earth completed its second lap, Mars would be back on the same side of the sun and a bit ahead of us along the track. The outcome of all this is that we end up on the same side of the sun as Mars every 2 years and 50 days.

There’s one more twist to all this. Our orbit is fairly circular, while Mars’ orbit is much more elliptical. Its distance from the sun varies quite a bit — between 206.7 million km and 249.2 million km, or about 20%. Every 15 years or so, the geometry works out that we pass Mars while we are near aphelion (the farthest distance from the sun) and Mars is near perihelion (its closest distance from the sun). When that happens, our two planets are at their minimum distance from one another. That’s what is happening this week. We have not been this close to Mars since 2003 and won’t be again until 2035!

When Earth passes a more distantly orbiting planet on the “inside track”, that planet appears in our sky opposite to the sun, a term astronomers call opposition. That planet will rise in the east as the sun sets in the west. And because we are closer to it, it will shine brighter than at any other date and look larger in binoculars or a telescope. The 2018 Mars opposition occurs at 1 am Eastern Daylight Time on Friday, July 27. At that time, Mars will shine brighter than any planet except Venus and show a disk that is 24.2 arc-minutes across. That’s about two-thirds of the size of Jupiter! A very good telescope, or a long exposure photograph taken through one, might even show Mars’ two tiny moons, Phobos and Deimos (Greek for Fear and Dread).

Because both Earth and Mars will be continuing to run their “race” after opposition night, and Mars’ true perihelion isn’t until mid-September, our actual minimum distance from Mars will occur on the night of Tuesday, July 31. At that time, the Red Planet will be 57.6 million km (or 0.385 astronomical units) from Earth. That translates to only 3 minutes and 18 seconds for radio signals to reach the planet, or vice versa.

(Above: The dramatic size difference in Mars’ disk between January 1, 2018 and closest approach on July 31, 2018.)

Here’s how to find and see Mars this week. On Sunday Mars rises at about 9:35 pm local time (depending on your latitude). It will climb until nearly 2 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. 20° is lower than many trees and buildings, so a clear southern vista is essential.

Since Mars is 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 by Friday night it will rise by about 9:15 pm local time, and the following Tuesday it will rise before 9 pm. Opposition occurs at 1 am EDT on Friday morning, so Thursday evening will be a fine time to look at it, too.

(Above: The arrangement of the sky at Mars opposition, shown for 1:25 am EDT on July 27, 2018. Hours earlier on the other side of Earth, the moon will have undergone an especially dark total lunar eclipse. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

If the clouds stay away, the planet will be impossible to miss. It will be brighter than anything nearby, except the moon. The 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 at opposition, even with a small telescope on a night of good seeing (i.e., with clear, steady air), you should be able to see Mars’ southern polar cap of frozen CO2 and water ice, and also some darker and lighter regions on the planet. Because 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.

(Above: The poles, prime meridian,and equator of Mars, plus labels for some of the features it will exhibit on opposition night. Viewing the planet all night long in July will show you about one-third of Mars’ globe. Night Sky Chart made via Star Walk 2 iOS and Star Chart for Android.)

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 the close encounter, 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. Keep an eye out on the media for the many Mars viewing parties that science centres, universities, and amateur astronomy clubs will be hosting.

Astronomy Skylights for this week (from July 22nd, 2018) by Chris Vaughan.

Keep looking up to enjoy the sky! I love getting questions so, if you have any, send me a note.