The evening sky presents quite a show this month. The bright planets Venus and Jupiter shine prominently in the northwest as the sky darkens.
It’s best to look for Venus first. The brilliant planet shines at magnitude –3.9 but still lies in twilight. Fortunately, the inner world continues to move away from the Sun and thus appears slightly higher in the sky with each passing day.
It makes a fine sight below Orion the Hunter at the start of the month. As the planet slides eastward relative to the background stars during May, it reaches Gemini the Twins and approaches Jupiter by month’s end. On the 31st, a telescope reveals Venus’ 13″-diameter disk and 80-percent-lit phase.
Look higher in the sky and you’ll easily see magnitude –1.9 Jupiter. The giant planet also moves eastward relative to the starry backdrop, but at a much slower pace than Venus. This brings it closer to the horizon from one week to the next.
A telescope shows nice detail in the gas giant’s atmosphere, particularly during moments of good seeing. In mid-May, Jupiter’s equator spans 34″ while its polar diameter is 2″ less. This polar flattening arises from the planet’s rapid rotation and gaseous nature. Also keep an eye out for Jupiter’s four bright moons — Io, Europa, Ganymede, and Callisto — as they change positions from night to night.
The other three naked-eye planets line up in the eastern sky before dawn in early May. Mercury lies closest to the horizon. It stands 5° high 45 minutes before sunrise but should show up easily if you have an unobstructed view because it shines brightly at magnitude –0.8. Saturn and Mars appear above Mercury and point toward it. The inner planet shows a nearly full disk that measures 5″ across when viewed through a telescope.
Mercury reaches superior conjunction May 14 and returns to the western evening sky near month’s end. A line from Jupiter through Venus then aims to the innermost planet. Similar to its morning appearance, Mercury appears bright but low in the sky. A scope shows its 6″-diameter gibbous disk.
Back in the morning sky, Mars maintains its steady eastward progress against the stellar background. Its relatively quick motion nearly keeps pace with the Sun, however, so it doesn’t gain a lot of altitude. On May 31, it appears about 15° high an hour before sunup and shines at magnitude 1.3. Through a telescope, Mars presents a bland disk measuring just 4″ across.
Because Saturn orbits farther from the Sun than Mars, it moves much more slowly relative to the distant stars and thus gains more distance from our star and also extra altitude in the predawn sky. By the end of May, it rises before 3 a.m. local time and climbs 35° high at the start of twilight. Glowing at magnitude 0.8, the ringed world easily outpaces the stars of its host constellation, Cetus the Whale.
Wait until Saturn climbs well above the horizon before targeting it with your telescope. As May winds down, you’ll be rewarded with a stunning view of the planet’s 17″-diameter disk and a ring system that spans 38″ and tilts 8° to our line of sight. Also look for 8th-magnitude Titan, Saturn’s largest moon, and a trio of 10th-magnitude satellites.
The starry sky
Scan high in the west after darkness falls and your eyes will land on the constellation Pyxis the Compass. French astronomer Nicolas Louis de Lacaille introduced this obscure constellation in the 1700s, and it is often associated with the ship Argo. Lacaille divided the great sailing vessel into Carina the Keel, Vela the Sails, and Puppis the Stern, but did not formally include Pyxis in the partition. In the 19th century, John Herschel suggested replacing Pyxis with a constellation he called Malus the Mast, assigning it as part of the structure of Argo. The name did not last, however, and we still have Pyxis.
Something else that did not last is a constellation Johann Bode introduced in 1801 when he produced his celebrated Uranographia star atlas. Around the southern and eastern parts of Pyxis, Bode created Lochium Funis, the Log and Line.
The Log and Line has a connection with Pyxis because both were used for navigating at sea. While a compass indicates the direction of travel — in relation to Earth’s magnetic field — the log and line helped measure a ship’s speed. A sailor threw a wooden object (the log) into the sea, and a line attached to it then slid through a crew member’s hands. With knots tied at regular intervals along the line, sailors could count the number of knots that passed through his hands over a set period. This technique gave rise to speed being measured in knots, a unit that survives today. One knot indicates a speed of one nautical mile per hour.
On Bode’s Uranographia, the astronomer drew the log of Lochium Funis to the east of the stars that make up the body of the compass, which he showed inside a box. The log includes Epsilon (ε) and Lambda (λ) Pyxidis. The majority of the line appears south of the box, with many turns in it so it includes a number of faint stars.
In a way, it’s a pity that Lochium Funis did not survive. Often, inclusions of certain objects can open a window into history, in much the same way as collecting stamps can reveal a lot about the world.
Star Dome
The map below 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. May 1
8 p.m. May 15
7 p.m. May 31
Planets are shown at midmonth
