Mercury at its evening peak
Mercury never strays far from the Sun in our sky. The innermost planet follows a tight orbit around our star so even at its best, it hangs low in the west after sunset or low in the east before sunrise.
That’s why it’s worth targeting Mercury this month as it puts on its finest evening show of 2024, becoming conspicuous in evening twilight after midmonth. At greatest elongation July 22, the planet lies 27° east of the Sun and stands 13° high in the west-northwest an hour after sundown. It lies 4° west (lower left) of 1st-magnitude Regulus, Leo the Lion’s brightest star. Five days later,
Mercury passes 3° south (upper left) of the feline’s luminary.
The planet’s phase shows up nicely through a telescope as the month ends. On July 31, Mercury appears 9" across and 30 percent lit. The beautiful shape pops into view during moments of good seeing.
As July winds down, look directly below Mercury and you should spot Venus. On the 31st, the inner planet hangs 5° high 45 minutes after sunset. Fortunately, it shines brilliantly at magnitude –3.9 and should show up easily if you have an unobstructed horizon. Venus doesn’t look like much through a telescope, displaying a nearly full disk that spans 10".
Once the two inner planets sink below the horizon, you won’t have long to wait for Saturn to rise. The ringed planet pokes above the eastern horizon in late evening. Saturn shines at magnitude 0.8 against the relatively dim backdrop of eastern Aquarius.
For the best views through a telescope, wait until the planet stands well clear of the horizon after midnight. Saturn’s flattened disk measures 18" across the equator in mid-July and stands out particularly well because the rings appear nearly edge-on to our line of sight. Their tilt increases slightly, from 2.0° to 2.4°, during the month. Also keep an eye out for Titan, Tethys, Dione, and Rhea, the ringed planet’s four brightest satellites.
Earth’s Moon occults
Saturn on July 24, but the only Southern Hemisphere spot offering a good view is Madagascar. From the city of Antananarivo, Saturn disappears behind the waning gibbous Moon’s bright limb at 18h32m UT; start observing a few minutes earlier to see the rings and some moons being occulted. The planet reappears on the dark limb at 19h17m UT.
Mars rises about four hours before the Sun and appears prominent in the northeast as twilight begins. Shining at 1st magnitude, it makes a welcome addition to the returning stars of winter. The Red Planet crosses the border from Aries into Taurus on July 11. Four days later, it slides 0.6° south of 6th-magnitude Uranus. Use a telescope to see both planets and compare Mars’ ruddy color with Uranus’ blue-green hue.
By month’s end, Mars appears to the left of the Bull’s head. With your naked eye or binoculars, compare the planet’s color with that of the red giant star Aldebaran. A telescope reveals Mars’ 6"-diameter disk but little else.
As July comes to a close, brilliant Jupiter stands 7° to Mars’ lower right. Gleaming at magnitude –2.1, the giant world easily outshines all the other planets and stars in this region.
Jupiter has now moved far enough from the Sun that it’s worth viewing through a telescope. During moments of good seeing, you should see fine atmospheric detail on its 35"-diameter disk. The planet’s four Galilean moons show up though any scope.
The starry sky
With the advent of telescopes in the early 17th century, astronomers began to discover more and more fuzzy patches of light in the night sky that were invisible to the naked eye. French astronomer Charles Messier (1730–1817) compiled the most famous catalog of these objects, which included many that other observers had discovered and one, the Pleiades Cluster (M45), that’s so bright it shows up easily without any optical aid.
But Messier wasn’t alone. Swiss astronomer Jean-Philippe Loys de Chéseaux (1718–1751) — who is best known for his co-discovery and observations of the Great Comet of 1744 — compiled a catalog of 21 objects, several of which he could see to be star clusters.
De Chéseaux observed from Lausanne, Switzerland, just a few kilometers inland from the shore of Lake Geneva. Surprisingly, 13 of the objects on his list lie south of the celestial equator despite his viewing from 46° north latitude. He even briefly mentioned Omega Centauri, which was permanently below his horizon, saying it had been “discovered by Mr Halley.”
De Chéseaux divided his list into two parts: one contained objects he discerned to be star clusters and the second that he classed as “the nebulae properly so called.” Only seven objects made the latter section. One of those was the great northern globular cluster M13 in Hercules the Strongman while three others were the southern globulars Omega Centauri, M4 in Scorpius the Scorpion, and M22 in Sagittarius the Archer. Clearly, the faint individual members of these globulars were beyond the capability of his small refractor, so they appeared as fuzzy patches.
Those three southern globular clusters rank among my favorite deep-sky objects. All three ride high in July’s evening sky, so take some time to look at them with your telescope on the next clear moonless night. Then imagine being in de Chéseaux’s shoes, and having a view likely similar to yours through a finder scope.
HOW TO USE THIS MAP
This map portrays the sky as seen near 30° south latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
The all-sky map shows how the sky looks at:
9 P.M. July 1
8 P.M. July 15
7 P.M. July 31 Planets are shown at midmonth
MAP SYMBOLS
Open cluster Globular cluster Diffuse nebula Planetary nebula Galaxy
STAR MAGNITUDES
Sirius 0.0
1.0
2.0 3.0 4.0 5.0
STAR COLORS
A star’s color depends on its surface temperature.
The hottest stars shine blue
Slightly cooler stars appear white Intermediate stars (like the Sun) glow yellow Lower-temperature stars appear orange
The coolest stars glow red
Fainter stars can’t excite our eyes’ color receptors, so they appear white unless you use optical aid to gather more light
SW
NW
BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT
N
SE
Note: Moon phases in the calendar vary in size due to the distance from Earth and are shown at 0h Universal Time.
CALENDAR OF EVENTS
1 2 3 6 7 12 13 25 27 28 29 30 31
The Moon passes 4° north of Mars, 18h UT
The Moon passes 4° north of Uranus, 10h UT
Neptune is stationary, 3h UT
The Moon passes 5° north of Jupiter, 8h UT
Earth is at perihelion (152.1 million kilometers from the Sun), 5h UT
New Moon occurs at 22h57m UT
Dwarf planet Ceres is at opposition, 0h UT
The Moon passes 3° north of Mercury, 19h UT
The Moon is at apogee (404,362 kilometers from Earth), 8h11m UT Jupiter passes 5° north of Aldebaran, 7h UT
First Quarter Moon occurs at 22h49m UT
The Moon passes 0.9° north of Spica, 3h UT
Mars passes 0.6° south of Uranus, 9h UT
The Moon passes 0.2° north of Antares, 20h UT
Asteroid Pallas is stationary, 21h UT
Full Moon occurs at 10h17m UT
Mercury is at greatest eastern elongation (27°), 7h UT
Pluto is at opposition, 6h UT
The Moon is at perigee (364,917 kilometers from Earth), 5h41m UT The Moon passes 0.4° north of Saturn, 21h UT
The Moon passes 0.6° north of Neptune, 15h UT
Mercury passes 3° south of Regulus, 12h UT
Last Quarter Moon occurs at 2h52m UT
The Moon passes 4° north of Uranus, 18h UT
The Moon passes 5° north of Mars, 11h UT
The Moon passes 5° north of Jupiter, 0h UT
Southern Delta Aquariid meteor shower peaks