As April kicks off, be sure to check out the predawn south-southeast sky for a beautifully close and colorful pairing-off of Mars and Saturn. They'll be joined by the moon on the 7th and then grow noticeably farther apart as the month progresses. Venus slowly climbs higher in the western twilight sky during April, while Jupiter begins to call attention to itself to evening skywatchers as it rises progressively earlier each evening. Finally, Mercury, which enjoyed the company of Venus in March, sweeps into the morning skies of April, but is low and rather difficult to see even at its greatest elongation near month's end.
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. Here, we present a schedule below which provides some of the best planet viewing times as well directing you as to where to look to see them.
Mercury – is in inferior conjunction on April 1st, passing between the sun and the Earth. In the days that follow, the innermost planet is too faint to be glimpsed at dawn until mid-April. By April 29th, Mercury reaches greatest western elongation, 27 degrees from the sun. However, as viewed from mid-northern latitudes, this will be the most unfavorable morning apparition of 2018 because it will positioned well to the south of the sun. This shy planet shines at +0.5 magnitude and is only about 10° above the horizon at sunup and rises less than an hour before the Sun.
Venus – Earth's "sister planet" gets a little higher each week during April. It remains small and roundish in telescopes this month, but from early April until early September, Venus will be at least 10 degrees above the horizon 45 minutes after sunset (though never very high), and during that period we will see its disk grow and enter its crescent phase. On April 17th, soon after sunset, look low toward the west-northwest for a view of a slender 2-day old crescent Moon, just 5 percent illuminated. And situated about a half-dozen degrees to its upper right will be the steady, dazzling light of the planet Venus. On April 24th, wait at least an hour after sunset to see the Pleiades star cluster poised beautifully about 3½ degrees to the upper right of Venus (binoculars help). Aldebaran and the Hyades are somewhat farther to the upper left of Venus.
Mars – is rising earlier and growing brighter as Earth moves up on it from behind. In April, it rises at around 2:15 a.m. local daylight time at the start of the month; closer to 1:30 a.m. at the end. Mars moves along its orbit only 3½ miles per second slower than Earth, on the average. It always tries to outrun us, but we always catch up, placing Mars at opposition every 2.14 years on average. By the end of April, Mars will have closed to within 79 million miles (127 million km) of Earth and will have doubled in brightness to magnitude -0.4. And it will grow more than nine times brighter by the end of July! Early in April Mars will team up with Saturn and later the moon for an eye-catching show (See Saturn below for more details).
Jupiter – starts April rising about 90 minutes after the end of evening twilight, but it comes up earlier and earlier each week. As soon as it clears horizon obstructions in the east-southeast, it grabs the attention of any skywatcher. Telescopic views of Jupiter during April are best in the middle of the night, when the planet has gotten at least moderately high. On April 2nd, very late in the evening, look low toward the east-southeast horizon about an hour before midnight and you'll see a waning gibbous Moon and 8 degrees directly below you'll find "Big Jupe." Through the remainder of the overnight hours of Apr. 2-3, the moon will appear to slowly approach Jupiter at roughly one moon width per hour. Then on April 30th, about an hour after sunset, look low to the east-southeast horizon for the rising of the moon, now one night past full. And about 5 degrees to its upper right, you'll again see Jupiter; the second time it has gotten together with our nearest neighbor this month.
Saturn — rises around 2:15 a.m. local daylight time in early April, and around 12:15 a.m. at month's end. Three hours later it is shining more than 20 degrees up in the southeast – providing an eerily beautiful end for a late-night telescopic observing session. Saturn's rings are still tilted enough this year (25.5 degrees at midmonth) to show above the planet's north pole and cover its south pole. Because Saturn is still two months away from opposition, it casts its shadow westward onto the rings. Check out the southeast sky a few hours before sunrise on April 2nd, as Mars slips 1.3 degrees to the south of Saturn. The two planets are all the more eye catching because they appear at practically the same in brightness (Mars is magnitude +0.3, while Saturn is just a trifle dimmer at +0.4), while also presenting a striking color contrast: Saturn is a baleful yellow, while Mars glows a fiery orange-red Both planets will be shining just to the upper left of the top of the lid of the Teapot in Sagittarius. On April 7th, although Mars and Saturn have noticeably separated since their pairing off five days ago, they are now joined this morning by the moon, just one day from its half or last quarter phase. The Mars-Saturn-Moon trio are arranged diagonally in that order, from lower left to upper right spanning 5 degrees. Mars and Saturn are separated by 3 degrees, while Saturn and the moon are separated by 2 degrees.
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.