A pair of planet conjunctions

August begins with dramatic evening views of the southern Milky Way arching high across a Moonfree sky. Before the sky darkens completely, however, look low in the west for the solar system’s two inner planets.

Mercury glows at 1st magnitude in the deepening twilight. On August 1, it stands 14° high 45 minutes after sunset and should be easy to spot. A telescope reveals its 9"-diameter disk and one-quarter-lit phase.

The innermost planet loses altitude with each passing day as it heads toward inferior conjunction August 19. As Mercury dips lower, brilliant Venus rises to meet it. You won’t mistake this world for anything else — at magnitude –3.9, it far outshines every other point of light in the sky. The two planets are in conjunction with each other on the 6th, when Mercury slides 6° south (upper left) of its brighter neighbor. Intriguingly, both worlds then span 10" when viewed with a telescope, but Mercury appears only 18 percent lit while Venus is nearly full (95 percent lit).

The Moon’s presence makes August 6 even more spectacular. From Australia, the slender crescent stands 4° to Venus’ upper right and 7° to Mercury’s right. As an added treat, the 1st-magnitude star Regulus lies just 2° to Venus’ lower left. All four objects will appear in a single field of view through typical binoculars.

Not long after Venus sets in the west, Saturn rises in the east. The ringed planet shines at magnitude 0.7, far brighter than any of the background stars of eastern Aquarius. Saturn slides 0.8° north of 5th-magnitude

Chi (χ) Aquarii in mid-August.

The planet looks remarkably different through a telescope than it has in previous years. The rings tilt just 3° to our line of sight at midmonth, a far cry from the wider opening we’re accustomed to. They currently span 43" as they wrap around the planet’s 19"-diameter disk. The narrow tilt of Saturn’s rings means the world’s moons stand out better than usual. A 10-centimeter instrument easily reveals 8th-magnitude Titan and its 10th-magnitude cousins: Tethys, Dione, and Rhea.

A waning gibbous Moon occults Saturn on August 21. The only Southern Hemisphere locations with good views are parts of Peru, Bolivia, and Brazil. From Arequipa, Peru, Saturn disappears behind

Luna’s bright limb at 0h50m UT and reappears on the dark limb at 1h34m UT. Locally, this is the evening of August 20. Remember to add a few extra minutes to these times to see the Moon gradually cover and uncover the rings.

The other two naked-eye planets pair up in the morning sky. In early August, Mars and Jupiter form a beautiful equilateral triangle with 1stmagnitude Aldebaran, Taurus the Bull’s brightest star. Mars shines at magnitude 0.9, nearly indistinguishable from the background star, while magnitude –2.1 Jupiter easily beats them both.

Although Mars and Jupiter trek eastward relative to the background stars of Taurus during August, Mars moves much faster thanks to its smaller orbit around the Sun. This sets up a close conjunction between the two. On the 14th, the Red Planet passes 0.3° north of its companion in front of the starry backdrop of the Bull’s long horns.

A telescope delivers startlingly different views of the two worlds. Mars appears only 6" across and shows little if any detail. However, the Sun illuminates 88 percent of its Earthfacing hemisphere, near the minimum possible. Can you detect its gibbous shape?

On the other hand, Jupiter boasts a wealth of detail. The giant planet spans 37" in midAugust, a large canvas for its turbulent atmosphere to cover. Look for two dark equatorial belts, one on either side on a brighter zone that coincides with Jupiter’s equator. Finer features show up during moments of good seeing. Also keep an eye out for the planet’s four bright moons as they change positions from night to night.

The starry sky

The French comet-hunter Charles Messier (1730–1817) compiled the most famous catalog of galaxies, nebulae, and star clusters. Published in the Connoissance des Temps for 1784, his final catalog included 103 objects. (Though note that M40 is just a double star and M102’s identity, which many observers accept as being the galaxy NGC 5866, seems more likely to have been a duplicate observation of M101.)

Look at the Messier Catalog today, and you’ll find it contains 109 objects (110 if you erroneously include NGC 205). This happened because astronomers added several objects later — much later, between 1921 and 1960. Messier or his friend and fellow observer, Pierre Méchain (1744–1804), discovered M104 through M109. (Indeed, Méchain and others first viewed many objects on Messier’s main list.) Of the new entries, American-Canadian astronomer Helen Sawyer Hogg (1905–1993) added M105, M106, and M107 in 1947.

At a declination of –13°, globular cluster M107 is the southernmost of the six additions to Messier’s catalog. It lies in Ophiuchus the Serpentbearer just 2.7° south-southwest of the magnitude 2.6 star Zeta (ζ) Ophiuchi and climbs high in August’s early evening sky.

The 8th-magnitude cluster appears as a fuzzy patch in 7x50 binoculars, though it appears far nicer through a telescope. A 15cm instrument reveals some of its stars, while larger apertures offer progressively better views. Enjoy hunting down this late-winter wonder.