Lunar secrets? How to see the moon’s hidden seas tonight

The combination of the slow rocking back and forth of the moon called libration brings into view three lunar maria or “seas” that are normally hidden around the backside – Mare Humboldtianum, M. Marginis and M. Smythii. To find them, you can use the easy-to-spot Mare Crisium. Credit: Virtual Lunar Altas

If everything revolved in perfect circles and all planet and moon orbits were concentric, the solar system wouldn’t be nearly as much fun. Consider the moon. Orbiting in a circle rather than ellipse,its distance from Earth would never vary. There’d be no “super moons” or full moons at the time the moon is closest to the Earth.

Simulated views of the Moon over one month, demonstrating librations in latitude and longitude. Credit: Tom Ruen

The moon’s orbital speed would also be constant and never get out of sync with its rotation rate. Because the moon moves slower when farthest from the Earth (and faster than average when closest), we can peer around the east and west limbs of the moon for a few days each month to see craters and lunar seas that are otherwise hidden. This apparent rocking back-and-forth of the moon, called libration, exposes an extra 7.9 degrees of lunar longitude for our viewing pleasure.

Similarly, if the moon’s orbit were exactly concentric with Earth’s and the moon’s axis straight up and down, we’d never be able to peek over and under its north and south polar regions. We’re grateful that the combination of the 5.1 degree tilt of the moon’s orbit and the 1.5 degree inclination of its axis exposes an extra 6.8 degrees of latitude. As you might guess, this tippy business is called libration of latitude.

Add in 1 degree of diurnal libration caused by our changing perspective at moonrise vs. moonset, and altogether we’re able to see 59% of the moon. Pretty cool, eh?

You can see the effects of libration tonight through next week if you have a pair of 10x binoculars or small telescope.

Here’s what the moon will look like on Feb. 14 when it will be full. Because of libration, two of the three featured lunar seas have now disappeared behind the moon’s eastern edge. Credit: NASA

Three lunar seas that normally are absent or appear as little more than skinny stripes along the extreme eastern edge of the moon are in good view this evening – Mare Humboldtianum (Sea of Humboldt), Mare Marginis (the Border Sea) and Mare Smythii, a sea named in honor of 19th century British astronomer Admiral Smyth.

Watch in the coming nights as the rock n’ rollin’ moon whisks them away.

Mercury and Venus trade places, liven up dawn and dusk this week

Venus is back! This time in the morning sky during mid-twilight. This view shows the planet about 10 degrees high (one fist at arm’s length) 40 minutes before sunrise tomorrow. Stellarium

Like resurrected gods, Mercury and Venus passed closest to the fiery glare of the sun earlier this month and disappeared from view for a time. Now they’ve returned but on opposite sides of the sun – Mercury in the evening and Venus in the morning.

Venus lingered for months in the dusky dusk until Jan. 11 when it passed between the sun and Earth and disappeared in the solar glare. Now she’s west of the sun, rising at dawn and visible with the naked eye about 40 minutes before sunrise.

Mercury has also stayed “close” to the sun and hidden from northern hemisphere sky watchers’s eyes during the first half of January. Now it’s crawling up the southwestern evening sky and will gradually become easier to see in the next week. While a feeble replacement for brilliant Venus, the solar system’s elusive, innermost planet is a rarer sight by far.

Mercury’s position shown 30 minutes after sunset on three dates. A very thin crescent will pass near the planet on Jan. 31. Mercury’s altitude is about 4 degrees tonight, 8 deg. on the 26th and 10 deg. on the 31st. The map shows the sky facing west-southwest. Stellarium

I’ve prepared simple maps for you to find both of these wanderers. You’ll need a clear horizon to the southeast to spy Venus and the same to the southwest for Mercury. You’ll find Venus much easier to spot because it’s so much brighter than Mercury and somewhat higher too.

In early January Venus lay east of the sun in the evening sky with its horns pointing left; now in late January, it’s swung west of the sun into the morning sky with the tips pointing west. Illustration: Bob King

If you’ve been following the planet over the last month, you’ll notice that the crescent is reversed from its evening appearance, with horns pointing up to the right instead of left. Tomorrow morning the crescent measures 59 arc seconds across or nearly one arc minute (60 arc seconds), equal to 1/30 the diameter of the full moon. 7-10x binoculars will easily show the delicate Venusian crescent.

Venus and Mercury on Jan. 19 viewed through a telescope (or in Venus’ case, also binoculars). Although not shown to scale in this illustration, Venus is more than 10 times larger in appearance than Mercury. Stellarium

Don’t expect to see Mercury’s humpbacked gibbous phase in the old opera glass. The munchkin planet spans only 3,032 miles across (2.5 times smaller than Venus) and is currently on the far side of its orbit 4.6 times farther from Earth than Venus. With a disk just 5.5 arc seconds across, Mercury’s phase will require a telescope magnifying around 100x to see clearly.

Now all you’ve got to do is resurrect yourself from the couch and go out for a look.

Venus at her finest today, but oh so shy

Despite its great elongation from the sun, Venus remains low in the southwestern sky during evening twilight for observers in mid-northern latitudes. Credit: Bob King

Demure Venus. Here’s a planet, brightest and arguably the most beautiful in the sky, that’s been hiding for months. It’s out there alright, but you better have a clear, open view to the west. For skywatchers living at mid-northern latitudes, which includes much of the U.S., Canada and Europe, Venus stands only about 10 degrees high (one fist held at arm’s length) in the southwestern sky a half hour after sunset.

Today it reaches its greatest elongation from the sun in the evening sky of 47 degrees  - as far away as it can possibly get. But you’d hardly know it. The planet of love and beauty has been hiding near the horizon for months. It’s not Venus’ fault.

Because the angle of the ecliptic to the evening horizon is shallow this time of year, most of Venus’s distance from the sun is nearly horizontal to the horizon with little of it going toward height or altitude. Maps: Stellarium

In the fall of the year, the path taken by the planets, sun and moon, called the ecliptic, makes a very shallow angle to the western horizon. Even though Venus is far from the sun, most of that distance is along the horizontal (to the east) with only a smidge in the vertical. After sunset, the rotating Earth makes quick work of putting the planet to bed.

In the spring, the ecliptic meets the horizon at a much steeper angle. When Venus reaches great elongation in April-May, most of that 47 degrees is straight up and vertical to the sun. Venus then shines in a dark sky for hours after twilight’s end, blazing like a sapphire.

Further south in Hawaii or Jamaica, the planetary highway (ecliptic) is tilted up more steeply. Venus follows suit and stands high above the horizon and sets later than from locations further north.

Of course I’m biased because I’m in my mid-forties – latitude that is. If you’re reading this from Jamaica or Hawaii you might be scratching your head. From those sunny locales, Venus shines in a pitch black sky and sets considerably later than in Cincinnati. How so?

Venus looks just like the first quarter moon in a small telescope. Look during twilight for the best view.

As you travel south along Earth’s curving surface, the angle of the ecliptic tilts ever upward, taking any planets along with it. Keep flying south beyond Jamaica to Lima, Peru and the ecliptic intersects the western horizon vertically at dusk.

All of those 47 degrees go toward lofting Venus high in the sky. The planet will make a stunning sight this coming month reflecting from the placid waters of the Caribbean and points south.

Venus reaches greatest elongation east (left) of the sun today on Halloween when its apparent distance from the sun is at maximum. In the coming weeks, Venus distance from the sun will decrease and its phase will shrink to a crescent as it slips between Earth and sun. Illustration: Bob King

Venus’s phase changes as it revolves around the sun interior to Earth’s orbit. Greatest elongation is a special time because the sun, Earth and Venus make a 90 degree angle to each other. From our perspective, Venus appears exactly 50 percent illuminated, looking just like a half moon sans pockmarks and spots. A perpetual cloud blanket covers the planet making it shine a fierce white.The best time for viewing the planet is during twilight, when sky light tames its terrific glare.

In the coming weeks, Venus will rise a bit higher in the southwest as the ecliptic angle increase slightly and the sun continues to set earlier. Come late November, watch for it to drop even lower as the angle it makes to Earth and sun narrows.

I should point out that while Venus hovers its highest in the sky for the next month, it’s not at its most brilliant. That occurs on Dec. 10 when it will shine at magnitude -4.6, slightly brighter than it is now. Even though the planet’s phase will only be a crescent then, its proximity to Earth more than compensates for its skinny profile.

In darkness the moon is reborn

If we could see the moon today, it would be a very thin crescent only a few degrees from the sun. Tomorrow it’s in new moon phase. Maps created with Stellarium

What’s old today but instantly becomes young again tomorrow? If you guessed the moon, you’re right! Today the moon winds up its current cycle of phases as an exceedingly thin crescent so close to the sun it’s invisible in the solar glare.

The moon’s cycle has always been a metaphor for life. Every month it’s born again as a thin crescent in the western evening sky, grows to a half-pie seven days later and reaches its full power and radiance when full at 14 days. After full, the moon’s radiance declines as its phase wanes to last quarter (21 days) and then to a whiskery crescent at dawn. Before it finally disappears in the sun’s glare, the moon, now 28 days old, reaches the end of its “life” cycle. But only briefly. The very next day, moments after new moon phase, it’s reborn again as an evening crescent.

When we run into troubles in our lives, we might look to the ever-renewing moon for inspiration.

The sky looking west-southwest a half hour after sunset Monday evening Feb. 11, 2013. You might be able to spot dimmer Mars in binoculars.

Tomorrow morning at 2:20 a.m. (CST) the moon will be exactly lined up with the sun and pass through new moon phase. Skywatchers in the western hemisphere won’t see the moon either day because it’s in the same direction as the sun and swamped by glare.

By Monday Feb. 11 however, the moon’s orbital motion will remove it far enough from the sun to be visible during evening twilight.  And there’s a bonus. The crescent will float a few degrees above the planet Mercury.

I’ve removed the atmosphere in this illustration so you can see where the moon is today at noon (CST), tomorrow morning when it reaches new moon phase and tomorrow Feb. 10 at noon. It passes north of the sun, which is in the constellation Capricornus. Notice all the planets in the neighborhood.

If we were to follow the moon today through new moon and into tomorrow, we’d notice it passes well north of the sun. Most of the time, the new moon is either north or south of the sun because its orbit is tipped about 5 degrees relative to Earth’s orbit.

The moon’s tilted orbit causes it to swing north or south (pictured here) of the sun from Earth’s perspective. A couple times a year however it crosses directly in front and a total solar eclipse is visible from somewhere on Earth. Illustration: Bob King

Over the course of its monthly cycle, it bobs up and then down along its tilted orbit. But 2 or 3 times a year, when the moon intersects the plane of Earth’s orbit at the same time as new moon phase, it crosses directly in front of the sun and we see a total solar eclipse. In fact, this is the only time we can see a new moon with the naked eye. It looks exactly like what you’d expect – a blank, black disk scrubbed free of its past life, waiting to begin the next as a tender crescent.

The new moon – black disk – is plainly visible silhouetting the sun during a total solar eclipse. Credit: Luc Viatour

Bye, bye Venus, see you on the other side

A colorful 22-degree halo, the most common variety, rings the sun yesterday afternoon. Like a prism, the ice crystals spread white light out into a rainbow spectrum giving the inner edge a red-orange tint and outer edge a pale blue. Photo: Bob King

A reminder to keep an eye out for halos around the moon and sun. We’re outside more often during the warmer months with more opportunities to notice things in the sky. Yesterday I caught the bottom edge of a colorful solar halo while making that first buzz with the lawnmower through a jungle of dandelions. Halos are created when light is bent or refracted by billions of microscopic, pencil-shaped ice crystals in cirrostratus clouds. Most have a radius of 22 degrees, making them 44 degrees or “four fists” in diameter. If you see thin, wispy clouds that stretch like a veil across the sky, halos may be in the offing.

Bye, bye Venus! Watch as Venus departs the evening sky in the next two weeks. These maps show the view facing northwest about 45 minutes after sunset. Created with Stellarium

Being outside in warmer weather also makes it easy to follow Venus’ quickening exit from the evening sky. Remember when it set at midnight? No more. 45 minutes after sunset tonight, the shining planet will be only 15 degrees high (a fist and a half held at arm’s length) in the northwest. A week from now that shrinks to 10 degrees and by late May, it’ll be lost in twilight’s glow.

Venus is a thin crescent through binoculars and small telescopes.

Venus is rapidly closing in on Earth, growing larger all the time. You should now be able to see the planet as a tiny crescent moon in 10 power binoculars. Make sure to focus sharply and observe in early twilight, when the planet’s glare is reduced. Venus undergoes phases just like the moon.

When it’s on the opposite side of the sun from Earth, it looks “full”, but as the planet catches up with ours, Venus’ phase shrinks to half and finally a crescent before gliding between the Earth and sun.  If you observe Venus regularly through the end of the month, you’ll see the crescent enlarge and grow thinner as the distance between the two planets shrinks.

Venus as a "reverse" crescent on June 15

After crossing the solar disk on June 5 during a rare transit, Venus will swing west of the sun and pop out days later in the morning sky at dawn. It will still be a crescent, but because it will then be on the other side of the sun as seen from Earth, the “left” or side of the planet will be illuminated as a thin crescent. The flip-flopping crescents are fun to see in binoculars and telescope; there’s no better demonstration of how Venus keeps on truckin’ along its orbit. As for the June 5 transit, I’ll have more on how to view it soon. It will be the last time in our lives we’ll be able to see one.

As Venus orbits, its changing angle with Earth and sun lets us see it in different phases like the moon. In the evening sky, the planet is to the left or east of the sun; when visible at dawn, it's to the right or west of the sun. Illustration: Bob King.

Moon pie on the menu tonight

The first quarter moon will cut a fine profile in tonight's sky. Created with Stellarium

It’s first quarter moon tonight. The moon’s completed a quarter of its circle around the Earth since new moon and will look like half a pie in the southwest in Sagittarius at dusk. It’s at this phase that the lunar craters really come into their own. Look along the day-night boundary called the terminator for a feast of rings and holes. That’s where the sun is rising on the moon. The interplay of long shadows and rim-lit crater walls from the low-angled sun create a fantastic landscape of dark depressions, sunstruck mountain peaks and richly-textured lunar seas.

The moon orbits the Earth in just over 27 days. The sun is at right. The moon forms a neat 90-degree angle with the Earth and sun at first quarter phase. Its angle to the sun continues to increase up until full moon, which is 180 degrees from or directly opposite the sun.

Binoculars will show the two basic varieties of lunar terrain: the ancient, white colored crust, which is saturated with craters from impacts that occurred some four billion years ago, and the smoother dark areas called seas or maria (MAH-ree-uh). These were created by even larger impacts, but later were filled up and smoothed over by lava welling up from the lunar mantle. By the time that happened – between 3 and 3.5 billion years ago – the asteroid and meteorite bombardment rate had declined, leaving the seas relatively less battered and smoother in comparison to the highlands.

A small telescope or binoculars will show craters especially well for several nights around first quarter phase. Three lunar mountain ranges are also shown. Photo: Bob King

It’s easy to see the seas in any pair of binoculars (or your eyes alone), and many craters along the terminator will show at 7x magnification. Certainly you’ll notice that the bottom or southern hemisphere of the moon near the terminator looks crinkly and rough from a bajillion impact craters. In addition to craters, the moon has its own mountain ranges, at least two of which you’ll see in binoculars and small telescopes tonight. The Alps (at right) might still be too shadowed by the terminator tonight to distinguish clearly except in a scope.

An interesting fact about moon phases is that it takes the moon 27.3 days to orbit the Earth, but it’s 29.5 days – almost exactly a month – between two identical phases. In other words, the time between successive full moons or first quarters is 29.5 days. Why is one longer than the other? During the time it takes the moon to complete a revolution around the Earth, the Earth-moon system moves partway around the sun.  Because of this orbital movement, the sun is in a slightly different place in the sky that during the previous month’s first quarter moon. We have to wait two additional days before the geometry is right for another first quarter moon.

First quarter to waxing gibbous phase is the best time to see so much of what the moon has to offer, because the terminator goes right down the middle of the moon. We face the craters and other features squarely and can literally look right inside them. Around crescent and full phases, the terminator is off near the moon’s edge causing features to become greatly foreshortened and stacked up one against the other. So enjoy a slice of the pie tonight any way you like it – eyes alone, with binoculars or magnified in a telescope.