See the Three Largest Worlds Between Earth & Jupiter
April 2014 :
One of the most notable changes to the evening sky during April is the accelerated pace at which the previous season's constellations become consumed by twilight as the Sun climbs higher along the ecliptic this time of year. Orion leans towards the west, and his belt becomes parallel to the horizon. Taurus takes a nosedive and the Pleiades put on their last good appearance. The stars marking the easternmost side of the Winter Hexagon (Procyon, Pollux, Castor, and Capella) form a long horizontal chevron which brackets the last vestiges of the winter sky sinking lower into the west with each passing night. Jupiter, which has taken residence among the twins of Gemini over much of the past year, remains the brightest beacon in the early evening sky. But as the hour becomes late, and the night air becomes still with a chorus of peeptoads in a distant pond, we turn our gaze towards the spring sky rising in the southeast. Leo is high overhead, the Beehive cluster and Coma Berenices are prominently positioned for binocular observing (and naked-eye if your skies are dark enough), and behind you the Big Dipper’s bowl begins to pour its stardust straight down onto Polaris, the North Star. At this hour we're looking out of the Milky Way's north pole, which is why the spring sky seems relatively sparse. This is, among other things, prime galaxy-hunting season. But for now we will focus our observations much closer to home.
Facing southeast, the familiar star patterns of Virgo and Corvus come into view. Spica, the brightest star in Virgo, has a visitor this spring. Ruddy Mars is making its best appearance since 2012 in our skies this month as it reaches opposition on April 9. Shining at a brilliant magnitude -1.4, Mars contrasts nicely with the magnitude 1.0 blue giant (spectral class B) Spica just below it. Because we are closest to Mars in April, and Earth is moving along in its orbit faster--and therefore overtaking--Mars, it is worth noting how the position of Mars changes with respect to Spica each night you observe.
Given that Mars is a rather small planet at roughly half the size of Earth, and that we are only close to it for a few weeks every two years, this is a long-awaited yet brief opportunity that planetary observers are eager to bring their telescopes out for. There are many things that make Mars a compelling and captivating object to observe, and an entire newsletter could be devoted to the topic of Mars observation, but for this tour we'll leave that to other sources.
The next planetary object beyond Mars may not be what first comes to mind. Yes, Jupiter is the fifth planet in our solar system, but well inside of Jupiter's orbit we have the asteroid belt. Unlike some cartoonish depictions of a densely packed wall of rocks, the asteroid belt is simply defined as a zone of the solar system between the orbits of Mars and Jupiter that is populated by bodies far smaller than the familiar planets. Even though there are many thousands of objects in this zone, the sheer size of the asteroid belt means that you would rarely see one asteroid from another. If the belt were packed as tightly as is often portrayed, it would be awfully difficult for us to see the outer planets from Earth. A few objects in the belt, however, are large enough to be seen from Earth using small telescopes or binoculars, and one may even be visible to the naked eye (given optimal conditions). For our tour this month, we will be visiting two solar system objects that reside in the asteroid belt and are often overlooked, yet are relatively close-by and easy to observe with binoculars--Ceres and Vesta.
You may recall that Vesta was recently visited by NASA’s Dawn spacecraft, which mapped the surface of this giant asteroid from September 2011 through July 2012. Vesta is among the largest of the asteroids, at over 500 kilometers in diameter, although not quite spherical.
Vesta reaches opposition on April 13, when it will be at a distance of 1.23 AU (154 million kilometers) from Earth. At this distance, it is expected to reach a maximum brightness of magnitude 5.7, placing it easily within reach of binoculars and small telescopes. In fact, this is the same magnitude that Uranus shines at, so if you observed Uranus last autumn you're already familiar with what an object of this brightness will look like in your sky and optics. And if you're up for a challenge and have superbly dark skies, you may even try to spot it without any optical aid.
Vesta is moving retrograde (westward) through central Virgo throughout April, and even though it is a bit farther away than Mars, you can still easily watch its progression from one night to the next. Be sure to watch it on the night of April 25th, when it passes about 3 arcminutes (1/10 lunar diameter) from the 5th magnitude star 78 Virginis. This encounter would be worth taking out your telescope for, as you will likely be able to perceive its motion with respect to the star.
The last stop on our tour is the dwarf planet Ceres. Ceres is the largest object in the asteroid belt, but because it is a bit more distant from Earth (1.64 AU at opposition) it shines more dimly than Vesta. But, at 7th magnitude, Ceres will be only slightly more challenging to spot than Vesta. Use the chart provided to track Ceres as it progresses through Virgo just east of Vesta.
As you are gazing upon this 1,000 kilometer-wide object, consider how, following its discovery in 1801, it was initially thought to be a planet. After all, it was discovered only 20 years after Uranus, and still 45 years before Neptune. It would soon be “demoted,” however, when more and more objects were discovered in its realm (sound familiar?), and became the first of what we now call “asteroids” (meaning “star-like”), describing their appearance in even the largest telescopes. However, Ceres would get promoted to dwarf planet status by the same decision by the International Astronomical Union in 2006 that resulted in Pluto being reclassified as a dwarf planet.
And, speaking of dwarf planets, it is exciting to report that next year will be the year of the dwarf planet, as both Ceres and Pluto will be visited by NASA spacecraft. The Dawn probe will enter an orbit around Ceres in February 2015 for an extended study, and we will learn more about its recently discovered water vapor plumes. Distant and enigmatic Pluto will get a brief visit by New Horizons as it speeds past in July 2015.
Keep watching Ceres and Vesta over the coming months as the apparent distance between them shrinks, until they are only two arcminutes apart on July 13. There is also a beautiful conjunction involving the Moon and Mars on July 5 that, along with Spica, Ceres and Vesta would make a striking photo.
