The Solar System in April

The Planets

Mighty Jupiter is sinking closer to the western horizon with each passing April evening. On the 1^st, it sets over two and a half hours after the Sun; by the 30^th, that interval is down to just 45 minutes, and – with the planet that low - effective observations are impossible. The take-away message: observe Jupiter while you can; it will soon be gone. At magnitude -2.1, the planet remains brilliant enough. You will no doubt be able to make out its 34”-wide disk and as many of its four major moons as may be in view, but observing any amount of detail on Jupiter’s disk will be challenging.

Aside from the Moon, Venus is the most brilliant object visible on April evenings, and doesn’t set for more than 3.5 hours after the Sun. As the month begins, the planet blazes at magnitude -4.5 high in the W. During April’s first few days, it makes a spectacular pass through the Pleiades star cluster; see our “What’s New:“ page for details. By April 30^th, Venus has grown even brighter, and reaches magnitude -4.7 – its peak during this evening apparition. (Don’t be surprised if there is a spate of “UFO” reports this month!). If you look through a telescope, however, you will see an increasingly larger but narrower crescent as the planet goes through its phases. On the 1^st, it presents a disk 25” across and 48% illuminated; by the 30^th, the disk is 37”’ across and 28% illuminated. After April, the disk will increase in size and continue to narrow in diameter until … well, for that, you will have to read next month’s Sky Report!

Mars is high overhead on early April evenings. It is about a month past opposition, so the planet dims from magnitude -0.7 to -0.1 during the month. Its disk - never large to begin with – shrinks from 12.6” to 10.0” across, but still remains large enough to present detail under good seeing conditions.

Saturn reaches opposition on April 15^th; since it is then “opposite” to the Sun in our sky, it rises as the Sun sets, stays up all night, and sets at sunrise. The geometry is such that Saturn is also closest to the Earth on this night, and attains its maximum magnitude of +0.2. A telescope will show the planet’s disk as 19” across. The real star of the show, of course, is the ring system, now 43” across, with its north side tilted toward us by 14”. With a decent instrument, it is possible to resolve the rings into the outer A ring and the inner, broader, and brighter B ring. In between is the hair-thin gap of the Cassini Division. The faint C ring, even closer to the planet, requires a larger instrument to resolve. (In case you’re wondering, yes – there are more rings: the D, E, F, and G rings have been discovered, but are only visible in observatory-class telescopes or from spacecraft in the vicinity of Saturn.)  Another attraction not to be missed is the satellite system. Titan, at magnitude 8.4, is visible in almost any telescope. Larger backyard instruments will show Rhea (magnitude 9.7), Tethys (10.2), Dione (10.4), and possibly Enceladus (11.7). A curious case is that of Iapetus, which has a hemispherical albedo dichotomy such that is shines at magnitude (10.2) when it is at the western elongation of its orbit around Saturn, and dims to magnitude 11.9 at its eastern extreme. This is a change of brightness by a factor of 5! 

Neptune rises about two and a half hours before the Sun by the end of April. The 8^th-magnitude planet shows a disk about 2.2” across.

Uranus climbs slowly out of the pre-dawn twilight during April. By the end of the month, it is rising an hour and a quarter before the Sun. A telescope shows it as a 6^th-magnitude disk about 3.4” across.

Mercury reaches greatest elongation (27° W of the Sun) on April 19^th. Due to the tilt of the ecliptic to the horizon in the spring, this is not a favorable morning elongation for Mercury viewers. Nevertheless, if you spot it low in the E before dawn on that date, you will see a magnitude +0.3 speck; a telescope shows it as a half-illuminated disk about 8” across.

Dwarf Planets/Asteroids:

Pluto, in NW Sagittarius, rises about four and a half hours before sunrise, and can be hunted down in the SE sky before dawn given a dark sky and adequate instruments.   

Ceres passes through solar conjunction on April 26^th and will not be visible this month.  

Vesta passes through solar conjunction on April 9^th and will be too close to the Sun to observe this month.