Face north after darkness falls and the prominent constellations Taurus, Orion, Canis Major, and Gemini will grab your attention. Mars resides in the last of these star groups during April’s first half. The magnitude 0.4 planet starts the month 4° south of Gemini’s luminary, 1st-magnitude Pollux. Mars’ eastward motion relative to the background stars carries it into Cancer on the 13th, and it ends the month shining at magnitude 0.9 near the center of the Crab.
Unfortunately, the ruddy world won’t look like much even through a large telescope. Its apparent diameter shrinks from 8.2″ to 6.6″ during April, too small to show much detail except during moments of exceptional seeing.
Although Mars remains prominent, it doesn’t hold a candle to brilliant Jupiter. The giant planet shines at magnitude –2.0 against the backdrop of Taurus the Bull, which lies lower in the northwest as night falls.
A telescope reveals Jupiter’s 35″-diameter disk highlighted by two dark atmospheric belts that run parallel to the gas giant’s equator. The planet’s low altitude likely precludes seeing much further detail. Any scope also shows Jupiter’s four bright Galilean moons.
Once Mars sets around midnight local time, the sky remains clear of naked-eye planets until the approach of dawn. That’s when Mercury, Venus, and Saturn push above the eastern horizon. The trio rises about an hour before sunup in early April and two to three hours before the Sun at month’s end. The three form a lovely triangle for early risers.
Venus dominates the scene. The inner world peaks at magnitude –4.8 in late April, shining some 100 times brighter than Mercury. Venus is pulling away from the Sun following its inferior conjunction in late March and grows even more dazzling as it climbs higher in a dark sky.
This is the time to observe Venus through a telescope. The planet’s apparent size and phase change noticeably from week to week. On April 1, Venus shows a disk 57″ in diameter that’s just 4 percent lit. As the month closes, the planet spans 37″ and the Sun illuminates 28 percent of its Earth-facing hemisphere.
Like Venus, Mercury passed through inferior conjunction in late March and grows increasingly prominent during April. The innermost planet reaches greatest elongation April 21, when it lies 27° west of the Sun and stands 14° above the eastern horizon an hour before sunrise. Mercury brightens slowly this month, achieving magnitude 0.1 by month’s end.
The planet’s telescopic appearance changes even more rapidly than Venus’ does. The beginning of April finds Mercury sporting an 11″-diameter disk that’s 7 percent lit. By month’s end, it appears 7″ across and 58 percent lit.
Saturn glows at magnitude 1.2 on the border between Aquarius and Pisces. Despite its low altitude, the planet offers a rare treat for telescope owners this month — the opportunity to see its glorious rings backlit. While Earth passed through Saturn’s ring plane in late March, the Sun doesn’t cross until early May. The best views should come late in the month once Saturn climbs higher in the predawn sky.
The starry sky
Although I have never seen a bird of paradise, I certainly hope to one day. Photographs show a magnificent if unusual creature, with males displaying long and elaborate feathers.
This makes viewing the constellation Apus, which represents the lovely bird, rather disappointing. Most egregiously, the bird’s tail feathers have been downsized. On April evenings, you can find Apus at the same altitude and to the left of the South Celestial Pole. The constellations Chamaeleon, Musca, Circinus, Triangulum Australe, Ara, Pavo, and Octans border Apus.
In February, I wrote about a group of four stars in Octans near the South Celestial Pole. This month, I want to highlight a different set of four, though close inspection reveals it to be a group of five because one turns out to be a naked-eye pair. The set comprises Delta (δ), Rho (ρ), Omega (ω), and Pi1 (π1) and Pi2 (π2) (the naked-eye pair) Octantis.
These four lie close to the border with Apus, centered at a declination of about –85° and roughly halfway between the pole and 4th-magnitude Alpha (α) Apodis. In Johann Bayer’s 1603 masterpiece, Uranometria, he included this group as part of the bird’s magnificent tail feathers. The book showed the tail spread out from the feet of the Chamaeleon (Beta [β], Delta1, and Delta2 Chamaeleontis) almost to the pole, but things had changed by the mid-18th century. Octans was now there, and Apus’ tail had to make way for part of the navigational instrument.
On Johann Bode’s Uranographia in the early 19th century, the tail feathers appear less than half as wide as before and actually narrower than the main part of the bird’s body.
In the early days after the introduction of Apus, the sky was less crowded. The lovely bird had room to spread its feathers before 18th-century French astronomer Nicolas Louis de Lacaille introduced 14 new constellations, of which Octans was one. Thus, the stars that proudly marked the middle of Apus’ tail feathers in 1603 are now officially part of Octans.
The lack of bright stars in this area makes it a joy to scan with binoculars. I particularly enjoy the view of the delicate pairing of Pi1 and Pi2 Oct.
Star Dome
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. April 1
8 P.M. April 15
7 P.M. April 30
Planets are shown at midmonth
