The new moon occurs Sunday (April 11), at 10:30 p.m. EDT (0230 GMT on April 12), a week before our natural satellite passes in front of Mars.
New moons occur when the moon is directly between the Earth and sun, and the two are at the same celestial longitude, a projection of the Earth's longitude lines on the celestial sphere. The position is called a conjunction. As the illuminated side of the moon faces away from Earth, unless there is a solar eclipse (in which the moon passes directly in front of the sun), they are invisible to earthbound observers.
Related: Best night sky events of April 2021 (stargazing maps)
Visible planets
As in March, the predawn sky contains the brightest planets. Observing from New York City (times will be similar in at mid-northern latitudes) the first planet to come up is Saturn, at about 3:39 a.m. local time on April 11, according to Heavens-Above.com calculations.
Jupiter follows at 4:16 a.m. Both planets are in the constellation Capricornus, and by about 5:30 a.m. Saturn will be about 17 degrees above the horizon and Jupiter about 13 degrees. Sunrise in New York City is at 6:23 a.m.
Mars, meanwhile, will grace the evening hours; the planet rises during the day at 9:30 a.m. in New York on April 11 and sets just after midnight at 12:36 a.m. on April 12. By 8:30 p.m. the planet is still 44 degrees above the horizon and its reddish cast makes it easy to spot; it will be one of the first objects visible after the sun goes below the horizon, along with the brighter stars.
Mercury and Venus, meanwhile, won't be visible — both are lost in the sun's glare and will remain so until next month, when Venus will emerge in the evening sky.
Those in darker-sky locations and using a small telescope or binoculars might catch the dim planet Uranus. South and to the right of Mars, Uranus is usually at magnitude 5 or 6, which is about the limit of visibility for the naked eye.
To find Uranus, you can trace a line between Menkar, the second-brightest star in the constellation Cetus, the whale, and Sheratan, the second-brightest star in the constellation Aries, the ram. The two stars form a triangle with Mars, and Uranus will be on the base.
Spotting Uranus in April will be challenging, as the planet is low to the horizon — by 7 p.m. on April 11 it is only 15 degrees above the horizon in the west and can be washed out by twilight.
The moon occults Mars
Some days after the new moon, on April 17, the moon will pass in front of Mars, an event called an occultation. It will be visible from Southeast Asia in a region stretching from eastern India, through Bangladesh, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore and Vietnam.
Occultations aren't visible from the whole night side of Earth because the moon is relatively close and its apparent position against the background stars differs depending on your location. This is called parallax, and you can use it to determine the distance to the moon, as early astronomers did (notably Hipparchus of Samos in 189 BCE). This is also the reason that solar eclipses are only visible from a narrow region on the Earth's surface.
For skywatchers in Ho Chi Minh City, for example, the occultation will begin at 9:22 p.m. local time, and the waxing crescent moon will be about 25 degrees up in the west-northwest, according to data from the International Occultation Timing Association (which also has a map of where the occultation is visible). Mars will disappear behind the unilluminated side of a waxing crescent moon and reappear on the illuminated side at 9:28 p.m. local time. As one moves roughly west the occultation occurs earlier; from Bangkok, the occultation starts at 8:14 p.m. local time and ends at 9:25 p.m. In Dhaka, Mars passes behind the moon at 7:59 p.m. local time and reappears at 9 p.m.
Farther west the occultation occurs in daylight. This is observable but dangerous to attempt because the moon will be relatively close to the sun in the sky; you should never attempt such daytime observations without taking precautions — even a split-second glance at the sun through a telescope or binoculars can permanently damage your eyes and cause blindness.
In Mumbai, the occultation begins at 5:32 p.m. India Standard Time, and ends at 8:49 p.m. The sun will still be up at the start; it sets at 6:56 p.m. local time. If you is looking for the moon during the day, especially when it is relatively "young," it is safer to start in the east and move your gaze west, as you will encounter the moon first with the naked eye. If you plan on observing this occultation in the daytime, seek help from experts!
Stars and constellations
By about 8 p.m. the sky will just be getting dark enough for the stars to come out; in New York City nautical twilight, when the sun is at 6 to 12 degrees below the horizon, starts at 7:59 p.m. on April 11. Sirius, the brightest star in the sky, will be in the southwest, with Orion to its right (west). The little dog, Canis Minor, marked by Procyon, will be above them both and slightly to the left, forming a rough parallelogram shape with Betelgeuse and Rigel.
Turning northwards, almost due west will be Capella, the brightest star in Auriga the Charioteer, which will still be high in the sky. To the right and below Auriga you can spot the "W" shape of Cassiopeia, the legendary queen of Ethiopia who boasted that she and her daughter were more beautiful than the Nereids. Since the boast made Poseidon angry, she was forced to sacrifice her daughter Andromeda, but Perseus saved her. The legendary hero is just to the left of Cassiopeia in the sky, and to her right is her husband king Cepheus.
Continuing towards due north, you will see the Big Dipper will be in the northeast, with the "bowl" nearly upside down, and the "handle" pointing to the horizon. The two stars at one end of the bowl are Alpha and Beta Ursae Majoris, respectively called Dubhe and Merak. Those point to Polaris, the North Star. Following those "pointers" in the opposite direction, you find Leo, the lion, which will be almost at the zenith (directly overhead) from mid-northern latitudes. The two stars at the back of the bowl point to the star Regulus, the brightest star in Leo. The more northerly star is called Megrez while the other is called Phecda.
If you think of Regulus as the front of Leo, with the sickle shape forming a head and mane, you can follow Leo's gaze along the zodiac, the constellations the sun passes through as it moves against the stars during the year. If you face roughly east so that Leo's head is to the right, you'll find Cancer, the crab, which is relatively faint and forms a trapezoid shape. Moving further to the right, you'll encounter Gemini, the twins, which will be high in the sky.
As the night progresses observers can see Virgo rise by about 10 p.m. The Big Dipper can help here; using the handle one can "arc to Arcturus" by drawing a sweeping arc to Arcturus, an orange-yellow star in Boötes, the herdsman, and then keep going to reach Spica, the brightest star in Virgo. Also rising in the northeast, just behind Boötes, is Corona Borealis, the Northern Crown.
In the Southern Hemisphere, sunset is getting earlier, as southern autumn approaches. In Buenos Aires, the sun sets at about 6:34 p.m. on April 11, and in Melbourne it sets at 5:59 p.m. local time. By 8 p.m. when it is fully dark the Southern Cross will be high in the southeast — from Melbourne, it is about 45 degrees above the horizon on April 12 (the new moon there occurs at 12:30 p.m. local time on that day). The Cross will be above Centaurus, where Alpha Centauri (otherwise known as Rigil Kentaurus) is, and turning East one can see Virgo rising (though "upside down" from the Southern Hemisphere). Continuing to turn north one encounters Leo, and then Cancer and Gemini as one's eyes continue towards due north.
Moving upwards from the Cross — towards the north, and the zenith — one finds the three constellations that make up Argo, the ship: Puppis, the deck; Vela, the sail; and Carina, the keel.
The Southern Cross can be used to locate south, much as the Big Dipper in the Northern Hemisphere can locate north. If you draw an imaginary line from the two brightest stars in the Southern Cross, which form the "vertical" post, extend it about four and a half times the length, and drops a vertical line to the horizon, one is pointed due south.
The South Celestial Pole is in the constellation Octans, the octant, a relatively faint triangle of stars — there's no equivalent of Polaris. So, another way to locate it is to use the "pointers" in Centaurus, Alpha Centauri and Beta Centauri. Centaurus is the constellation just below the Cross in the sky, and Alpha and Beta are its two brightest stars. If you draw an imaginary line from halfway between those two stars and another bright star, Achernar (the end of Eridanus, the river), the halfway point marks the pole.
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