David Aguilar (617) 495-7462
Christine Pulliam (617) 495-7463

Our Solar System: April 2014

The brightest point of light in the early evening sky is Jupiter. It should become visible shortly after sunset, and will appear to be almost overhead once darkness falls. Even from deeply light-polluted urban skies and under marginal sky conditions, Jupiter will be visible when little else is. It sets about 2:30 AM as April begins, and about 12:45 AM by month's end. Jupiter appears in Gemini. If you are more familiar with the unchanging patterns of constellations than with the planets, which constantly shift their positions with respect to the background stars, then you should find it easy to recognize the distinctive stars of Orion; if you draw a line from Rigel to Betelgeuse and extend it by one additional length, you will come to Jupiter. At midmonth, the planet is about 499 million miles away, and is slowly increasing its distance from us. Light from the sunshine illuminating its cloudtops takes about 45 minutes to reach us; we can thus say that Jupiter is now 45 light-minutes away. Even a small telescope reveals the two main dark belts - the North and South Equatorial Belts - on either side of the planet's equator. Larger scopes reveal numerous others. Jupiter is at eastern quadrature - 90° east of the Sun - on April 1, so any shadows Jupiter's four large Galilean moons cast appear far from the moons themselves. And you might see an unusual sight on April 21: though the moons are usually strung out in a line parallel to the planet's equator; for a short time around 11:15 PM EDT, Callisto, Io, and Europa form a line inclined about 45° to the planet's axis.

At every opposition - roughly every 26 months - Mars gets to be a star. This April is such a time. This is when Mars is at its closest and brightest, and draws the most attention. On April 8, the planet is directly opposite to the Sun in our sky, so it rises as the Sun is setting, is highest around midnight, and sets as the Sun is rising. If Mars' orbit around the Sun were more circular, opposition would also be the time the planet is closest to Earth. However, the Red Planet's orbit around the Sun is rather elliptical, so the two events do not necessarily coincide. This year, for instance, opposition occurs on April 8, but Mars' closest approach to Earth is on April 14. And just how far apart are the two planets? Their separation is about 57.4 million miles; even though relatively close as Solar System objects go, it still takes 5 minutes for its light to reach us. At its closest distance, Mars shines at magnitude -1.5, brighter than Sirius. In a telescope, it subtends an angle of 15.2" - large enough to show considerable detail to skilled observers under good conditions. It is important to note that not all oppositions of Mars are created equal. During the next several oppositions, Mars will be closer and larger; during the opposition in 2016, we'll get to see it as large as 18.4" across. Better yet, during the 2018 opposition the planet will swell to 24.1" across. This year, Mars' Northern Hemisphere is tilted toward us, so features such as the North Polar Cap will be well displayed. In spite of the fact that it is now early summer in the Martian north, and the carbon dioxide frost covering it is sublimating away, the water ice cap underneath never completely disappears. Some of the larger dark albedo features, particularly the more northerly ones such as Syrtis Major and Mare Acidalium, will be the ones most easily visible. And keep an eye out for clouds and possible dust storms.

After Mars, the next planet to come up is majestic Saturn. The Ringed Planet rises at about 10:30 PM local time as April begins, and about 8:30 PM by month's end. It is approaching opposition in May, so is not quite at its best, but next month it will only get marginally closer, larger and brighter than it is now. During April, it brightens slightly from magnitude +0.3 to +0.1; the angular diameter of the planet's disk grows from 18.2" to 18.6", while its visible rings swell from 41.3" to 42.2" across. Through a telescope, the rings are quite a sight; their tilt toward us of 22° to our line of sight shows off the detailed structure, beauty, and complexity of the ring system. In a moderately sized telescope, one can distinguish the bright B ring from the outer, dimmer A ring. Under good conditions, the dark hairline of the Cassini Division between these two rings may be visible. This in itself is an amazing fact; at midmonth, Saturn is 836 million miles away; yet the Cassini Division is only about 3,000 miles wide. Larger telescopes under ideal conditions reveal several more gaps in the ring system; flyby and orbiter space missions have revealed thousands. Also not to be missed are Saturn's wonderful moons. Titan, the largest, is larger than the planet Mercury, and has an atmosphere denser than Earth's! At magnitude 8, the moon is visible in almost any small telescope. The next set of smaller moons - Rhea, Dione, Tethys, and Iapetus - are in the range of 10th-11th magnitude, and require larger scopes.

The next planet to rise in April skies is Venus. It doesn't come up until after 4 AM local time, so it is now most definitely a morning "star." At magnitude -4.3, it is by far the brightest point of light in the night sky. As seen through a telescope, however, it can be a bit of a disappointment. When we look at Venus, we never see its surface. The planet is completely blanketed by a thick layer of bright clouds. While the features of the planet have been thoroughly mapped by spacecraft in orbit around it using radar, no one has ever seen its surface. What we can see, in even the smallest telescopes, are the phases of Venus. There can be a crescent Venus, a gibbous Venus, a full Venus, etc. At the beginning of this month, the planet's disk is 22" across and 54% illuminated; by April's end, it will have shrunk to a ball 17" across and 66% illuminated.

As viewed from our vantage point in the Solar System, the brightest planet in the sky, Venus, makes a close passage to the dimmest one: Neptune. In the early morning hours of April 12, Venus passes just 0.7° - a little over a Moon-width - north of Neptune. The latter will be difficult to see in the twilit sky; at magnitude 7.9, it will be dimmer than Venus by a factor of almost 70,000 times!

Like, Venus, Mercury is very low in the morning sky as April begins. It reaches inferior conjunction - passing "behind" the Sun - on the 26th.

Uranus reaches solar conjunction on April 2, and is too close to the Sun to be visible this month.

Dwarf Planets/Asteroids

The dwarf planet/asteroid 1 Ceres, in Virgo, reaches opposition on April 15. Now at its closest distance to us, it lies 152 million miles away, and shines at magnitude 7.0. On opposition night, it lies about 15° from both Mars and the Full Moon.

The asteroid 4 Vesta, also in the constellation Virgo, remains just 2.5° W of Ceres. Like Ceres, Vesta reaches opposition this month - in Vesta's case, on the 13th. The asteroid is about 115 million miles away, and shines at magnitude 5.8; this implies that, under dark skies, it should be well within the range of naked-eye visibility.


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