Skylights: July 2024

July is the month with the second-shortest amount of observing darkness.

Earth is at its most distant from the Sun, called aphelion, at 1:06am EDT on the 6th, when it will be 1.0167255287671 au. This is 3.4% farther away than we were in January.

On the 20th, the Sun enters Cancer, where it will reside for the next 21 days.

July begins with the Moon going through its waning crescent phase. On the 1st, it passes 4.7° east of Mars.

On the 2nd, the 14% illuminated crescent is 2.7° north of Uranus and 3.6° southwest of the Pleiades Cluster.

The Moon is new at 6:57pm on the 5th, marking the beginning of Lunation 1256. 

On the 7th, the waxing crescent Moon is 2.5° northeast of Mercury. On the 9th, it is 5.0° east of Regulus, in Leo.

First quarter Moon is at 6:39pm on the 13th. The brightest stellar occultation by the Moon, magnitude 1.0 Spica, in Virgo, happens on the 13th-14th. From our location, the star dips out of view at 23:24 and reappears 65 minutes later, when, unfortunately for us, it will be below the horizon..

The waxing gibbous Moon is 3.0° east-southeast of Antares, in Scorpius, on the 17th, and 1.0° north of dwarf planet Ceres on the 19th.

July’s Full Moon is known as the Buck Moon. The nearest moonrise to full phase occurs at 8:18pm on the 20th, the evening before it is full.

The Moon is full at 6:17am on the 21st in Sagittarius. Just before it sets at 5:13am, it passes in front of, or occults, a 4.7 magnitude star known as Terebellum (omega Sagittarii), the northwesternmost star in a kite-shaped asterism. The occultation lasts from 1:43am to 2:29am.

The Moon appears 4.0° east of Saturn on the 24th, and 3.7° southwest of Neptune a few hours later, on the morning of the 25th.

The Moon is last quarter at 10:57pm on the 27th, in Aries.

On the 29th, the waning crescent Moon is 4.8° west-northwest of Uranus; the following morning, it lies 3.8° east of the Pleiades.

Mercury is at greatest elongation, 26.9° east of the Sun, on the 22nd, but it will be in a better position to observe earlier in the month, with its latest setting time being at 9:43pm on the 8th, about 80 minutes after sunset.

The 6th-7th is a particularly interesting time to watch Mercury. Make sure to find a place with a clear western horizon and go out just 30 minutes after sunset. Mercury will be about 8° above the horizon, and at magnitude -0.2 should appear in binoculars despite bright twilight. On the 6th, look 10° to the right of Mercury, at about the 3:30 position, to find the 1.3% illuminated, 26-hour young crescent Moon. If you look early enough, and your horizon is low enough, you may spot Venus scintillating through the haze 4° below the Moon.

As the sky darkens, keep watch around Mercury, as some stars may begin to appear, as the planet lies directly in front of the Beehive Cluster, M44, in Cancer on the 6th. Many of the members of the cluster are 7th magnitude, so you’ll need a larger telescope with medium magnification to pick them out through the twilight. A bit later, Mercury moves lower, the sky becomes a bit darker, and the stars of the cluster should be easier to observe. Now look 1.4° to the north of Mercury to find magnitude 8.4 Vesta. You may even be able to fit them in the same field of view before they dip below the horizon. Vesta, at 3.359 au, is over three times as distant as Mercury, at 1.093 au.

On the 7th, Mercury no longer lies among the stars of the Beehive, but the crescent Moon is much better placed 2.6° northeast of Mercury, at about its 11:30 position.

Mercury spends the final days of the month within a few degrees of Regulus, in Leo, coming as close as 2.2° southwest of the star on the 25th, before sinking out of sight on its way to inferior conjunction.

Mercury appears as a small gibbous globe until the 18nd, when it shows a 7.4 arcsecond, 50% “half-moon,” and subsequently progresses through a crescent phase. On the 28th, it reaches 33% illumination at 8.8 arcseconds.

Venus emerges in the evening sky in July, with visibility extending from 30 to 50 minutes post sunset through the course of the month. Following Mercury, Venus passes in front of the Beehive Cluster in Cancer on the 17th, although at this late date it is unlikely you will be able to observe any of the cluster’s stars. Even Vesta will be invisible by the time Venus passes 1.6° to its south on the 23rd.

Venus closes in with Regulus during the final days of July, coming within 5.3° northwest of Leo’s brightest star on the 31st. On the same night, notice that Mercury and Venus lie the same distance above the horizon, and are nearly equidistant from Regulus, with Mercury being south-southeast of the star.

Telescopically, Venus doesn’t appear larger than a 10-arcsecond 96% gibbous during July.

Rising at 2:00am on July 1st, Mars appears 4.7° east of the 23.8-day waning crescent Moon, in Aries.

Mars enters Taurus on July 11, joining Uranus and Jupiter in the celestial bull. Mars pairs with Uranus on the 15th, when the two planets are separated by just 0.6° in our sky, but 18.44 au across the solar system, with Uranus being 12 times more distant and 1/80th the apparent brightness of Mars.

During the second half of July, Mars passes through the gap separating the Hyades and Pleiades clusters in Taurus, and incrementally approaches Jupiter. The 24.4-day waning crescent Moon is 4.0° north of Mars on the 30th.

Also on the 30th, note how Mars, Aldebaran and Jupiter form a near-perfect isosceles triangle, with the Aldebaran vertex being just over 6° from each planet. Additionally, this is a good time to compare the brightness and color of Mars and Aldebaran, both of which shine at a ruddy 0.8 magnitude.

Mars shows a 5.8 arcsecond globe that is 89% illuminated. Through a large telescope during steady seeing conditions, you may begin to see surface markings.

By the end of July, Mars rises just after 1:00am.

Jupiter resides in Taurus, and now rises into the sky before the onset of astronomical twilight. Through the span of July, Jupiter’s rising time ranges from 3:00am to 1:30am. The giant planet’s eastward motion is apparent by watching its changing position with respect to Aldebaran, the nearest bright star. In early July, Jupiter is within 5° north of the star.

On the 3rd, the 7.5% illuminated waning crescent Moon is 4.0° north of Jupiter.

Here are some notable arrangements of Jupiter’s four Galilean moons during July: 

The moons are arranged in order of orbital radius to the east of the planet on the 2nd. Io goes into Jupiter’s shadow beginning at 3:23am on the 7th. Io emerges from transit at 4:43am on the 8th. Io and its shadow transit Jupiter on the 15th, and with Europa in eclipse, just Ganymede and Callisto are visible close to Jupiter’s northwestern limb. Another arrangement of the moons in order of orbital radius, this time to the west of Jupiter, is visible on the 28th. Io goes into shadow at 3:36am on the 30th. Io and Europa go into transit on the 31st.

As the evening sky’s only bright planet, Saturn rises into view before midnight in early July, and is visible before 10:00pm at the end of the month.

With Saturn’s ring plane nearly aligned with Earth, the positions of its moons are now nearly linear with respect to the planet, similar to the appearance of Jupiter’s moons. With a medium-sized telescope, you should be able to easily find, besides its largest moon, Titan, the mid-sized moons Tethys, Dione, Rhea, and Iapetus. These moons orbit Saturn with periods of 1.9, 2.7, 4.5, and 79,0 days, respectively.

With the glare of the rings diminished, a larger telescope will be able to resolve the inner moons Mimas and Enceladus, which orbit Saturn in 22.6 and 32.9 hours, respectively.

The waning gibbous Moon joins Saturn on the 24th.

Uranus is located in Taurus, 5.6° south-southwest of the Pleiades cluster. The waning crescent Moon is 2.7° north of Uranus on the 2nd.

Mars lies 0.6° to the south of Uranus on the 15th, and is within the same binocular field for a week before and after that date.

Neptune, in Pisces, reaches its stationary point on the 2nd, and will appear to be moving westward (retrograde) until December 8. Observing the distant ice giant near its stationary point becomes slightly more difficult as its position among the background stars doesn’t change much on consecutive nights.

Through much of July, Neptune lies at nearly the same right ascension as omega Piscium. Aligning an equatorially mounted telescope on the star, then moving 8.2° to the south, one will find themselves within ¼° of the planet.

At magnitude 7.3, Ceres is at its best in July, reaching opposition on the 6th, in Sagittarius, at which time the dwarf planet will be at a distance of 1.888 au from Earth, and easy to locate within ½° north of Ascella (zeta Sagittarii).

On the 15th, Ceres is just ½° north of globular cluster M54.

The 97.6% illuminated waxing gibbous Moon passes just 1.0° north-northwest of Ceres at approximately 11:00pm on the 19th.

On the last evening of the month, Ceres is 1.7° north of globular cluster M70.

Just over an hour following Ceres, the other notable dwarf planet, Pluto, comes into view, in western Capricornus. Pluto is at opposition on the 23rd, at a distance of 34.046 au, and shining dimly at magnitude 14.4. As distant Pluto hasn’t moved much since last season, the globular cluster M75 can still be used as a guide in locating it, 2.2° to its southeast.

Comet 13P/Olbers, a periodic comet with a 67.9-year orbit, appears in the northwestern sky after sunset, and is bright enough to be visible in binoculars. It tracks along the Three Leaps of the Gazelle during July.

The comet is closest to Earth on June 30, at 1.175 au, and reaches perihelion, at 1.895 au, on the 20th. 

A pair of meteor showers coincide at the end of the month, when moonlight will not product much interference. The Delta Aquarids, which is active for a month from mid-July through mid-August, peak on the 30th, and can produce as many as 20 meteors per hour. The meteors originate from a point in the sky near delta Aquarii, also known as Skat, which rises around 10:00pm.

The Capricornids is a low-rate shower that is active through most of July and into early August. Its peak date is on the 31st, when the radiant, which lies a few degrees north of Algedi (alpha Capricorni), is above the horizon during the hours of darkness; however, the shower produces only about 5 meteors per hour.

Both of these showers overlap with the Perseids, which are active from late July through August.