An encounter for the ages
Early April finds the evening sky bereft of planets, and conditions don’t improve much as the month progresses. Although Mercury comes into view late in the month, this is a poor apparition. Even at greatest elongation from the Sun on April 29, the inner planet appears only 5° high in the west-northwest 30 minutes after sunset.
But the morning sky tells a different story. Venus, Mars, Jupiter, and Saturn gather in stunning fashion before dawn. And at the start of the month, Venus, Mars, and Saturn occupy a single binocular field in eastern Capricornus.
Venus shines at magnitude –4.4 and dominates the trio. It lies some 30° high in the east an hour before sunup with Saturn 3° above it and Mars 3° higher still. When viewed through a telescope, Venus’ disk spans 22" and appears 55 percent lit.
The inner planet’s eastward motion relative to the background stars carries it into Aquarius on the 3rd and Pisces on the 27th. This pace closely mimics the Sun’s own speed and, as a result, Venus maintains a similar predawn altitude all month. The outer planets move more slowly and thus gain elevation each day.
This sets up a series of encounters observers won’t want to miss. The closest comes April 27 when Venus meets distant Neptune. The two worlds come nearest at 19h12m UT, or 5:12 a.m. local time on the 28th in eastern Australia. Their centers approach within 25" of each other, which means their limbs stand only 15" apart. Although Venus’ glare makes spotting 8th-magnitude Neptune a challenge, the effort will be worth it.
Three mornings later brings a spectacular conjunction between Venus and Jupiter.
At around 19h UT on the 30th (5 a.m. local time May 1 in eastern Australia), the magnitude –4.1 inner world passes 0.23° — slightly less than half the Full Moon’s apparent diameter — south of the magnitude –2.1 gas giant. The two brightest planets haven’t been this close since August 27, 2016.
Jupiter’s visibility improves more than any other planet this month. It rises as twilight begins in early April but ends the month more than 20° high at the equivalent time. Like Venus, Jupiter has a close encounter with Neptune. At
20h UT on April 12 (6 a.m. on the 13th in eastern Australia), the giant world skims 6.5' due north of its sister world.
Of course, Jupiter looks great whenever you turn a telescope toward it. On April 30, its disk spans 35" and shows plenty of atmospheric detail. The most obvious features are two dark belts that sandwich a brighter zone coinciding with the planet’s equator.
Saturn also climbs significantly higher these autumn mornings. The magnitude 0.7 planet begins April near Venus but quickly leaves its brighter cousin behind. By month’s end, the ringed world stands nearly two-thirds of the way to the zenith as twilight starts to paint the sky.
The view of Saturn through a telescope never fails to impress. In mid-April, the planet’s disk measures 16" across while its splendid rings span 37" and tilt 13° to our line of sight. Under good viewing conditions, the Cassini Division that separates the outer A ring from the brighter B ring should be easy to see.
Don’t let Mars be an afterthought in your morning observing. It begins April in a tight grouping with Venus and Saturn and, like the other planets, moves eastward against the stellar backdrop. Mars travels at an intermediate pace, however, and quickly catches up with sluggish Saturn. The Red Planet slides 0.3° south of the ringed world the morning of the 5th. Unfortunately, Mars’ 5"-diameter disk shows no detail through a telescope.
A partial solar eclipse occurs April 30 for people in southern South America. From Santiago, Chile, the Moon blocks 40 percent of the Sun’s diameter at maximum just before sunset. Be sure to use a safe solar filter to view this event.
The starry sky
The great ship Argo rides high in the south on April evenings. This constellation proved so unwieldy that 18th-century French astronomer Nicolas Louis de Lacaille divided it into the three smaller constellations — Carina, Puppis, and Vela — we still use today.
If you look at some of the delightful old celestial charts illustrating the constellations, you might notice that Argo has no bow, with rocks or clouds sometimes replacing this section. The Greek poet Aratus suggested that the ship’s front end was lost “when Argo pass’d through Bosporus betwixt the justling rocks.” He was referring to the Planctae Rocks at the opening of what we now know as the Black Sea.
Whatever the reason for the ship’s damage, Edmond Halley used that end of the constellation to invent a new one: Robur Carolinum (Charles’ Oak). It represented the oak tree in which King Charles II hid in 1651 following the Battle of Worcester, in which Charles lost the final skirmish of the English Civil War to Oliver Cromwell.
In his 1801 masterpiece Uranographia, Johann Bode depicted the oak’s trunk as a line anchored by the stars Theta (θ) and Eta (η) Carinae, with u Car and several nearby dim stars forming the tree’s leafy parts. However, Halley made Beta (β) Car his Alpha (α) Roburis, giving the trunk greater heft. I think it’s safe to say that few stargazers today know that this rich part of the sky once contained a king’s hiding place.