The next full moon is only two days away (August 22nd at 6:02am MDT). The phase we see tonight is called the ‘waxing gibbous’ phase since it occurs after the first quarter phase and is moving toward full.

During the waxing gibbous phase, much of the Moon’s face pointing toward Earth is in lunar daylight. With the Sun high in the lunar sky, there’s not a lot of contrast between light and shadow, so what we see at this time are largely patterns of different shades… except on one part of the Moon. This is where my favorite lunar feature, the lunar terminator, comes into play.

The lunar terminator is the area of the Moon that separates day from night. During the waxing gibbous phase, the terminator marks the portion of the Moon that is experiencing sunrise (sunrise at a given location on the Moon occurs about every 27 Earth-days. This is because the Moon rotates once on its axis every 27.3 Earth-days – much slower than Earth’s rotational period of 24 hours).

From our vantage point on Earth, the lunar terminator slowly moves across the face of the moon from right to left. Shortly after ‘new moon’ (when the Moon is between the Earth and Sun and is not visible to us), we see a thin crescent moon in the western sky, shortly after sunset. The slim crescent is the portion of the Moon that we can see that’s in daylight (during the crescent phase, most of lunar daylight occurs on the far side of the Moon).

As the Moon moves in its orbit, the crescent gets wider, eventually becoming a first quarter moon. At first quarter, the right half of the Moon’s face is in daylight, the left half is in lunar night. Each night on Earth, the terminator (the ‘line’ between day and night), moves slightly to the left, eventually disappearing at full moon. After full moon, the sunset terminator appears on the right side of the Moon and marches toward the left. At this time, the Moon is in the waning gibbous phase, moving toward last quarter, the waning crescent phase, and then back to new moon.

Below are four photos of the waxing gibbous Moon, taken on consecutive nights in May 2020. If you look at each photo, the area of the Moon along sunrise terminator shows great sunlight and shadow contrast because the sunlight, as seen from the Moon’s surface, is low. Lunar features that are normally unremarkable stand out in vivid detail during this transition from night to day.

Photo 1. Waxing gibbous moon, taken May 2, 2020.

In Photo 1 above, taken on May 2, 2020, you’ll notice at the terminator, near the left edge of the photo, an arc of a crater wall that contains a flat area of lava (the crater wall looks like the letter C). The flat area is known as ‘Sinus Iridum’ or ‘Bay of Rainbows.’ Much of the crater wall surrounding Sinus Iridum is on the night side of the sunrise terminator, but it’s high enough to catch the light of the rising Sun, even though the lava plain below the wall is mostly still in night’s shadow.

Photo 2: Waxing gibbous moon, taken May 3, 2020.

On May 3, 2020 (Photo 2, above), Sinus Iridum and its surrounding crater wall are in full sunlight and new features have moved into view. For example, a large, round lava basin known as ‘Mare Humorum’ (the ‘Sea of Moisture’) has appeared at about the 6:30 position on the Moon. A younger crater named Gassendi, with a central peak, can be seen at the upper left of Mare Humorum. These features were in lunar night when Photo 1 was taken, but can now be seen in detail, given the interplay of light and shadow near the terminator.

Photo 3: Waxing gibbous moon, taken May 4, 2020.

On May 4, 2020 (Photo 3, above), the crater Gassendi is less obvious since the Sun is higher in its sky and the sunrise has brought other features, including the crater ‘Aristarchus’ into view. Aristarchus is the small, bright white crater, just inside the terminator, at the 8:00 position. Aristarchus is so white that it can be seen with the naked eye.

Photo 4: Waxing gibbous moon, taken May 5, 2020.

Finally, on May 5, 2020 (Photo 4, above), the sunrise terminator has moved far enough to bring the crater Grimaldi into view. Grimaldi is the large, dark-floored crater at the 7:00 position. It looks like the bottom third of a ‘snowman’ of craters that lies just inside the terminator.

Also, the large crater Clavius is now visible at the 5:00 position (it’s also about 5:00 from the bright, rayed crater Tycho). Clavius is the second largest crater on the visible side of the Moon. Because of the light and shadow contrast along the sunrise terminator, you can see at least one small crater on the floor of Clavius.

If you pick any feature in Photo 1 and find it in the following photos, you’ll notice that as the feature moves away from the terminator, it becomes less distinct as the contrasting light and shadows lessen. But, every night on Earth, the sunrise and sunset lunar terminators, with their low sunlight and deep shadows, bring new features into view, in sharp relief. A few moments observing along the terminator with binoculars or a telescope is a highly rewarding experience. Even if you do this regularly, you’re bound to find something you hadn’t noticed before.

The Perseid meteor shower is arguably the best meteor shower of the year. Under optimal conditions, 60 or more meteors per hour can be seen during the shower’s peak. The meteors are tiny particles of ice and rock, remnants of Comet 109P/Swift-Tuttle, whose orbit the Earth intersects every August.

While clouds hindered the viewing of the peak of the Perseid meteor shower this week, they didn’t ruin the show. Here are a few photos taken on three mornings this week.

August 7th: A faint Perseid slashes down toward the gate wall in the encroaching dawn.

August 12th: Close up of a Perseid fireball. This meteor’s brightness exceeded that of Venus as the cometary debris slammed into the atmosphere. The meteor started out in the red end of the spectrum (upper left) and transitioned to blue-white as it heated up and vaporized.

August 13th: Another Perseid appears above the gate, to the upper right of rising Orion (the meteor’s actually in the neighboring constellation, Eridanus).

August 13th: The meteor in the previous photograph left a ‘train’ that glowed for over 90 seconds. The glowing train is the result of vaporized rock and metal (likely iron and sodium), mixing with ozone in the upper atmosphere. This photo is a 30-second exposure, taken a few seconds after I saw the meteor. You can see the upper level winds are already beginning to contort its shape.

August 13th: This is the fourth and final photo of the meteor train (called a ‘persistent train’). It was a 20-second exposure, ending about 90 seconds after the beginning of the first meteor train photo (it was actually a few seconds longer since I paused between photos). If you compare this photo to the previous one, you’ll note that the train drifted (toward the bottom of the photo) against the background of the stars.

August 13th: Two meteors slide across the stars, one to the upper right of the gate, just below Orion’s belt, the other at the upper left of the photo. The meteor at the upper left is a ‘sporadic’ meteor – a meteor that’s not associated with the Perseid meteor shower.

The Perseid meteor shower is arguably the best meteor shower of the year (I’m partial to the Geminids, which can be viewed in the cold, clear skies of December). At the shower’s peak and under optimal conditions, you can expect to see about 60 meteors per hour, emanating from the constellation Perseus.

A Perseid meteor falls toward the bright stars Castor and Pollux in Gemini during dawn in 2018.

The Perseid meteors are particles cast off of Comet 109P/Swift-Tuttle. The shower is broad, so you can see Perseids every year, from July 17th until August 24th.

This year’s peak occurs on the night of Wednesday August 11th and predawn hours of Thursday August 12th. Several sources indicate that the shower peaks on the night of the 12th and morning of the 13th, but the American Meteor Society says the peak is on the 11th and 12th, so that’s what I’m going with. However, you should see ample meteors on both nights.

Conditions for watching the Perseids this week may be a mixed bag. The moon sets around 10pm MDT, so the sky will be dark all night. However, clouds and the continued layer of wildfire smoke that blankets New Mexico may interfere. Weather forecasts can be off, so don’t assume that clouds will hinder your viewing.

A faint Perseid slashes the sky above the garden gate on Saturday morning, August 7th.

While partly cloudy conditions can cut into the number of meteors seen, you can still view some, as the above photo shows (the sky that morning was about 40% cloudy). Find a spot with a clear view of the sky, lay back, and observe. Meteors can appear anywhere in the sky, so there’s no best place to look. If the sky’s partly cloudy, find the largest patch of clear sky and focus on that.

Enjoy the show.

The month of August offers incredible night sky events, weather permitting, of course.

The southwest monsoon season is in full swing this summer, compromising stargazing, but delivering much needed water to New Mexico. Photo taken July 6, 2021.

Saturn is in opposition to the Sun and closest to Earth on August 2nd. Jupiter’s opposition follows on the 19th. Both planets will be bright (Jupiter being the brighter of the two) and visible all night.

The Perseid meteor shower, one of the best showers of the year, peaks on the night of August 11th and pre-dawn of the 12th. With the waxing crescent moon setting around 10:00pm MDT, observing conditions will be perfect for viewing about a meteor per minute, weather permitting.

‘Stargazer’ watches a Perseid meteor explode as it slams into the Earth’s atmosphere. Photo taken August 12, 2020, Santa Fe, NM.

The Summer Milky Way is well-positioned for evening viewing on clear, dark nights. As darkness falls in early August, the Milky Way will span the sky from the southern to the northeastern horizons. By month’s end, it will have shifted a bit, reaching from the SSW to the NNE horizons, passing almost overhead. Darkness occurs about 40 minutes earlier than in early August, making the Milky Way visible earlier in the evening.

August Planets:

Mercury opens August behind the Sun. By month’s end, it’s visible during evening twilight in Virgo, setting an hour after the Sun. Since the ecliptic is tilted toward the southwest, Mercury doesn’t get very high during this evening apparition.

Venus begins the month easily visible in Leo. By the end of the month, it moves higher in the sky and shifts east, into Virgo, setting as night begins (about 9:05pm MDT).

Mars begins August near Regulus, in Leo, but it’s almost on the opposite side of the Sun from Earth and is thus faint and hidden in twilight glare all month.

Jupiter reaches opposition to the Sun and closest to Earth on August 19th. See ‘August Spotlight’ for further details.

Saturn reaches opposition on August 2nd and is visible all night in the constellation Capricornus. This month is the best time to view Saturn for the remainder of the year since it’s close to Earth and its rings are wide open.

Jupiter (the bright ‘star’ on the left) and Saturn (2:30 from Jupiter, above the horse sculpture) follow the Summer Milky Way into the night sky, as they both approach opposition in August. Photo taken July 8, 2021.

August Spotlight:

The planet Jupiter has attracted attention since humans began gazing at the night sky. It’s usually the second brightest object in the night sky, excluding the Moon (Venus is the brightest; once every 15-17 years, Mars can briefly outshine Jupiter).

Jupiter is exceedingly bright now because it’s almost at its closest distance to Earth. Opposition (the time when the Sun, Earth, and planet line up, with Earth in the middle) occurs on the 19th of August. At that time, Jupiter will shine in Aquarius at magnitude -2.9. The next few weeks offer our best chance to view Jupiter until it next reaches opposition in September 2022.

To the unaided eye, Jupiter shines like a brilliant white pearl, rising around sunset and setting around sunrise. A small telescope will reveal Jupiter’s four largest moons, called the Galilean Moons. These four large moons were discovered and determined to be satellites of Jupiter by Galileo Galilei, when he turned his new telescope on Jupiter in 1610. I find it rewarding to watch these moons nightly, since they slowly change position relative to Jupiter and to each other.

Jupiter and its four Galilean moons. Photo taken with a Canon digital camera and 600mm lens.

Ganymede, the largest of the four moons, is slightly larger than the planet Mercury. Calisto is smaller than Mercury, but much larger than Earth’s Moon. Io is about the same size as our Moon, while Europa is a bit smaller.

Since Jupiter orbits the Sun just beyond the asteroid belt, its gravity has captured a number of small asteroids that strayed too close to the giant planet. Currently, there are 79 known moons of Jupiter, all but the four Galilean Moons being small and beyond the reach of small telescopes.

Sometimes, Jupiter captures objects and gobbles them up. One well-observed instance occurred in the early 1990s, when a comet was gravitationally captured by Jupiter. The comet broke apart before colliding with the planet, leaving a series of black disturbances in Jupiter’s upper atmosphere.

The dark splotches in Jupiter’s atmosphere mark the sites where pieces of Comet Shoemaker-Levy 9 slammed into Jupiter’s atmosphere. The Great Red Spot is visible at the lower left. Photo by NASA Hubble Telescope and copied from scientificamerican.com.

If you look closely at Jupiter through a small telescope, you’ll notice that it’s a bit wider at the equator than it is at the poles. This is due to the rapid rotation of the planet. Even though Jupiter’s diameter is about 11 times Earth’s, it spins once on its axis (a.k.a., a day) in just under 10 hours, versus 24 hours for an Earth day. This rapid rotation causes Jupiter’s atmosphere to noticeably bulge at the equator.

You’ll also observe faint horizontal bands crossing the planet. These are cloud patterns at the top of Jupiter’s atmosphere, caused by the planet’s rapid rotation and high winds. While most of Jupiter’s atmosphere is composed of hydrogen and helium, the visible clouds at the top of the atmosphere are mainly ammonia. The dark colored atmospheric bands are called ‘belts;’ the lighter colored ones are ‘zones.’

Detailed photographs of Jupiter’s weather at the top of the atmosphere, taken with telescopes and satellites, reveal these belts and zones to be incredibly complex swirling patterns and colors. One atmospheric feature that can be seen in a small to medium-sized telescope is a reddish oval called the ‘Great Red Spot.’ The Great Red Spot is a storm that has been continuously observed from Earth for over 300 years.

A close-up of Jupiter’s dynamic atmospheric belts and zones, photographed by NASA’s Juno orbiter.

Some astronomers consider Jupiter to be a ‘failed star’ since its atmosphere is mainly composed of hydrogen and helium, similar to the Sun, but lacks sufficient mass and pressure for the process of atomic fusion to occur. While true, the planet developed from the accretion of rocks and ice, like other planets in the solar system, not the compression of gas and dust that leads to star formation, like the Sun.

Thus, the solar system’s largest planet shines by reflected sunlight, rather than by light produced by internal thermonuclear reactions. Regardless, Jupiter is a breathtaking sight, whether viewed with the unaided eye or a telescope. Enjoy observing the planet and its four largest moons while it’s close to Earth (a mere 365 million miles at its closest on the night of August 19th).

August Night Sky Events:

August 1-31: The Zodiacal Light is visible about an hour before dawn. The Zodiacal Light is sunlight from the not yet risen Sun reflecting off of interplanetary dust particles. Clear, dark skies are required to see this faint light.

Zodiacal light glows over Galisteo before dawn, June 30, 2020. Venus is rising in the center, below the Pleiades star cluster. Mars is the bright object in the upper right quadrant.

August 2: Saturn is at opposition to the Sun and closest to Earth, visible all night long in the constellation Capricornus.

August 8: New moon.

August 11-12: The Perseid meteor shower peaks on the night of the 11th and morning of the 12th. This could be a great shower this year since the Moon sets around 10:00pm on the 11th. At peak, the shower could produce in excess of 60 meteors per hour.

August 19: Jupiter is at opposition to the Sun, shining at magnitude -2.7, near the Capricornus-Aquarius border.

August 22: Full moon occurs at 6:02am MDT. Since this is the third of four full moons in a three month period, it’s known as a ‘blue moon’ (as in, “once in a blue moon”). Blue moons occur about every 2.7 years.

An almost full moon rises before sunset. Distant wildfires spewed small particles into the atmosphere, contributing to the Moon’s faded appearance and yellowish color. Photo taken July 22, 2021.