What is an asteroid? Is one likely to strike the Earth? Asteroids are solar system debris ranging in size from Ceres, nearly 580 miles in diameter, down to grains of sand. The asteroid belt is located between the orbits of Mars and Jupiter, and several hundred thousand are known today, with many of the largest ones tracked by programs such as NASA’s Near Earth Object Program.

The most likely origin of the asteroid belt is that it represents debris that failed to coalesce into a planet during the formation of the solar system. This is a more likely scenario than the alternate theory, that the asteroids are the remains of a destroyed planet, since their total mass is less than the Earth’s moon. Many of the asteroids have orbits that cross Earth’s orbit and the possibility of a collision between us and an asteroid gives rise to alarmist headlines in the media on a regular basis.

Asteroids of roughly the size of a football field may hit the Earth every 1,000 years, while monsters up to a half-mile in diameter may impact every several millions of years. One of the latter size, landing in the Yucatan Peninsula, is thought to have caused the extinction of the dinosaurs about 65 million years ago. The latest “near miss” was by an asteroid roughly 300 feet in diameter that passed between the Earth and moon in 2002. The good news is that increasingly sophisticated tracking devices are lessening the possibility of Earth being hit by an unknown asteroid. The bad news is there is little or nothing we can do about an asteroid impact.

Focus on the planets

Mercury is low on the eastern horizon about a half-hour before sunrise all month. Well to the lower left of Venus, Mercury will continue to brighten during the month but still remains obscured in the glow of dawn.

Venus rises in the east at dawn as April opens. Venus is still the brightest object in the sky at dawn but continues to dim as its rapid orbital motion carries it away from us.

Mars is high in the west as darkness falls but continues to fade and grow dim as the distance between the red planet and Earth continues to grow.

Jupiter rises in the southeast shortly before midnight as April opens and remains in the sky nearly all night. By the end of the month, the giant planet rises at sunset and is high in the south by midnight. Watch the dance of Jupiter’s four major moons as they circle the planet.

Saturn is high in the southwest at sunset and remains in view until nearly dawn as the month begins. Castor and Pollux, the Twins, are the bright stars just to the upper right of Saturn. Viewers with telescopes should be able to spot details such as the space separating the two major ring systems and Saturn’s shadow falling on the rings just to the east of the planet.

Uranus may be spotted by telescope on April 18 immediately to the south of Venus. The pair are situated low on the eastern horizon about 5:30 a.m. with Uranus appearing as a tiny blue-green disk.

Neptune lies in the constellation of Capricornus, just west of Uranus in Aquarius, where it is lost in the glow of dawn.

April events

1 Sunrise, 5:17 a.m.; sunset, 6:02 p.m. Look to the west about 45 minutes after sunset to see the moon pass very near the Pleiades star cluster.

2 This is the first Sunday in April and time to put your clocks ahead by one hour as the nation goes from Standard Time to Daylight Saving Time for the summer. Sunrise, 6:15 a.m.; sunset, 7:04 p.m.

3 Mars lies just below the moon tonight. The bright star far to their right is Capella, the “Goat” star in the constellation of Auriga.

5 Moon in first quarter, 8:01 a.m. Shortly after dark this evening the moon is very near Castor and Pollux, with Saturn to the moon’s left.

6 Saturn lies just below the moon as darkness falls.

9 The moon is at apogee, or farthest distance from Earth, today.

13 Full moon, 12:41 p.m. The full moon of April is called the Egg Moon, Milk Moon, or Grass Moon.

16 Easter Day. Easter is the first Sunday after the full moon after the vernal equinox.

19 The sun enters the constellation Aries on the ecliptic.

20 The sun enters the astrological sign of Taurus even though, astronomically, it has just entered Aries.

21 Moon in last quarter, 11:28 p.m.

22 This is the peak night for the Lyrid meteor shower. Originating out of Lyra the Lyre, high up on the eastern horizon after midnight, you can expect to see up to 20 meteors per hour with no interference from the moon.

24 Look to the east at dawn to see Venus paired with the waning crescent moon.

25 Moon at perigee, or closest approach to the Earth, today.

27 New moon, 3:45 p.m.

30 May Eve, a cross-quarter day marking the midpoint between the spring equinox and summer solstice. Sunrise, 5:27 a.m.; sunset, 7:39 p.m.

Send astronomical queries to Clair Wood at cgmewood@aol.com..