With Venus now gone from our western evening sky, we must now direct our attention after sundown to the east for a view of a different brilliant planet: the mighty Jupiter.
A worthy successor to dazzling Venus, Jupiter will become the new prominent "evening star" right on through the spring and summer. Meanwhile, over in the west we find a fading Mars, and for the first 10 or so days of April a much brighter Mercury will glow well below and to the right of Mars. Saturn rises in the middle of the night and when dawn breaks it's reaching its highest point in the south, just as Venus — now a resident of the morning sky — pops up into view above the eastern horizon.
In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10 degrees. Keep reading below to find out how to see some of the best planet sights of April 2017, times as when and where to look to see them.
Mercury – is an evening "star" setting soon after dusk. On April 1 it reaches its greatest elongation, when it is 19 degrees east of the sun and is almost directly above it at sunset. This is the planet's best evening appearance of the year for observers in north temperate latitudes. In mid-twilight (45 minutes after sundown), Mercury is still 10 degrees above the western horizon. Look quick, however, because within 10 days Mercury fades to invisibility as it passes through its crescent phases and slides back toward the sun. Mercury passes through solar (inferior) conjunction on April 20, but remains too dim to see before dawn, even by month's end.
Venus – shines very low in the dawn. This planet's steep plunge out of the evening sky and sudden vault into the dawn heavens were spectacular in March and now it begins April rising less than an hour before the sun. But by month's end the interval is closer to 1 3/4th hours. During these upcoming weeks, the always-brilliant planet kindles even more, reaching a peak brightness of -4.7 by the end of the month. At mid-month its crescent will thicken from its ultra-thin sliver of almost an arc minute long to 13 percent illuminated though also appearing about 16-percent smaller. And by month’s end those figures have changed to 27-percent and one-third smaller. On the morning of April 23, if you look low to the east-southeast about 45 minutes before sunrise you’ll see a waning crescent moon and well to its upper left, Venus will dazzle.
When Venus appears fully illuminated (or nearly so), it's positioned on the far side of the sun as seen from Earth and through telescopes its disk appear quite small. Conversely, when Venus is near its closest point to the Earth, its disk appears six times larger, but it appears as only a hairline filament of light. The morning of April 30 marks the compromise: Venus is at its "greatest illuminated extent." Its disk appears relatively large in apparent diameter and is just over one-quarter illuminated. Also, Venus is now at the pinnacle of brightness for this morning apparition, outshining Jupiter nearly eight-fold and appearing more than 20 times brighter than Sirius, the brightest of all stars.
Although Venus remains low at dawn all month at mid-northern latitudes, observers in the Southern Hemisphere see Venus soar dramatically higher during April; viewers in Buenos Aires, Cape Town and Melbourne see Venus’s altitude at sunrise go from 1° on the 1st to about 34 degrees by the April 30!
Mars – low and just north of west at dusk, passed from Aries into Taurus on April 12. It's now readily identified as the only bright object well to the right of Aldebaran. The reddish planet is more than half a magnitude dimmer than the similarly-colored star. By May 6, this second-magnitude planet will be 6 degrees north Aldebaran. Mars currently sets about 2 1/4th hours after the sun and after the end of twilight. On the evening of April 28, a 9-percent waxing crescent moon sits about 3 degrees to the upper left of Aldebaran and both are positioned well to the left of Mars.
Jupiter – comes to opposition on April 7, rising in the east near sunset and remaining visible all night long. Around opposition Jupiter is at its closest to the Earth for the year lso brightest and biggest in telescopes. This year, however, happens to be the one when Jupiter is nearest to aphelion, the farthest point from the sun in its 12-year circuit. (Jupiter reached aphelion on Feb. 16). But Jupiter is a giant planet in a rather circular orbit. That means that even at this farthest opposition Jupiter still shines at a generous -2.5 magnitude, less than a half magnitude dimmer and about 12-percent less wide than at its closest oppositions.
If you wait until late evening to observe Jupiter on nights this month, you will find at least a few darkish horizontal belts in small telescopes and a plethora of details on its cloudy face in medium-size (6-or 8 inch) instruments. During April, Jupiter drifts northwest in Virgo, moving away from the 1st-magnitude star Spica. On April 10, the moon, just hours from turn full, lies only a couple of degrees to the lower left of Jupiter as the two rise in the east-southeast as darkness falls this evening.
Saturn – peeks up over the southeast horizon around the middle of the night. By dawn it's high in the south. Saturn is still more than two months from its June 15th opposition, but in a telescope it’s already as large as it will get. This year in telescopes, the Saturnian rings remain tilted into view by no less than 26 degrees, almost their maximum possible amount.
When Saturn is this far from opposition it casts its shadow sideways on the rings, enhancing the lovely three-dimensional look of the whole arrangement. During the predawn hours the planet is evident appearing as a bright yellow-white "star" shining far to the lower left of the waning gibbous moon. On the following morning (April 17), the moon will have shifted its position relative to Saturn, appearing well off to the left of the ringed planet.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers’ Almanac and other publications, and he is also an on-camera meteorologist for Fios1 News in Rye Brook, NY. Follow us @Spacedotcom, Facebook and Google+. Originally published on Space.com.