SKY REPORTER: April 2011
by Steve Beyer on
Friday April 1st the Sun is within boundaries of the constellation Pisces. In New York City that morning civil twilight begins at 6:12 a.m. Eastern Daylight Time followed by sunrise at 6:40. The following evening sunset is at 7:20 and civil twilight concludes at 7:48 p.m.
On the last day of the month, civil twilight begins at 5:26 a.m. with Sunrise arriving at 5:56. The Sun sets that evening at 7:51, followed by the end of civil twilight at 8:20 p.m. Our nearest star’s direction then is within boundaries of the constellation Aries. During April, time between sunrise and sunset each day increases by one hour 15 minutes.
Primary lunar phases in April are: New Moon, Sunday April 3rd at 10:32 a.m.; First Quarter, Monday the 11th at 8:05 a.m.; Full Moon, on Sunday April 17th at 10:44 p.m.; and Last Quarter, at 10:47 p.m. Sunday April 24th.
This month average overnight temperatures recorded in Central Park at Belvedere Castle near the American Museum of Natural History rise from 40 degrees Fahrenheit on the first to 49 degrees on the 30th.
For most of this month the planet Mercury is invisible to direct view from Earth due to its position amidst solar glare. At the end of April it reemerges in the predawn eastern sky and may be seen in morning twilight by experienced observers.
Mercury is currently being studied via the Messenger Mission spacecraft that went into orbit on March 18th around this closest planet to the Sun. Messenger's primary science phase was scheduled to begin in early April. Check the following NASA website for updates and images:
Venus is about 117,031,000 miles from Earth on April 1st, when it rises at 5:23 a.m. The planet reaches an altitude of about 8 degrees of arc above the horizon towards the east-southeast by the start of civil twilight. It then may then be seen 13 degrees to the right of the thin waning crescent moon. Venus remains a vivid sight on clear mornings throughout this month. By April 30th the planet’s distance from us increases to about 132,711,000 miles.
Until late April Mars and Jupiter are, along with Mercury, hidden from our view by the Sun’s glare.
During April Saturn is located in the direction of stars in the constellation Virgo, and remains an easy object for viewing. This beautiful planet, with the largest ring system in the solar system, is now well placed for observations throughout the night. During the night of April 3rd and 4th, it will be directly at the opposite side of Earth from the Sun, a celestial event aptly called “opposition”. Distances between Earth and Saturn, as well as Mars, Jupiter, Uranus, and Neptune are at their minima during times of opposition. At this month’s opposition Saturn is about 800,721,000 miles from us.
On the evening of April 7th, the waxing crescent moon is located between the Pleiades and Hyades open star clusters in the constellation Taurus. Such groups are called open clusters because their member stars eventually all fly apart as they gradually escape from each other’s gravitational attraction. However these clusters may endure for many hundreds of millions of years before their stars disassociate. The Hyades is in the same apparent direction of the first magnitude star Aldebaran, although it is not a cluster member. The Pleiades cluster is also known as M45, indicating its placement in Charles Messier’s list of sky sites.
The open cluster Messier 35 in the constellation Gemini is two degrees from the wide crescent moon on the night of April 9th. Both objects are near the location along the ecliptic occupied by the Sun on the Summer Solstice, June 21st. M35 is about 2800 light years from the solar system, has an age of 150 million years, and contains about 2500 stars. In nearly the same direction from us is another open star cluster known as NGC (New General Catalogue) 2158. It has a distance of 11,000 light years and the approximate age of a billion and a half years. Stars of the latter cluster are more concentrated than those in M35.
Tuesday April 12th the waxing gibbous moon is seven degrees of arc south of Messier 44, also known as the Beehive or Praesepe open star cluster. Although under clear dark skies M44 is one of the few open clusters visible to unaided eyes, binoculars are always provide extra enjoyment when observing it and other clusters of stars. The several hundred stars of the Beehive cluster are estimated to have a distance from the Solar System of 550 to 600 light years, and an age of about 730 million years. Both its distance and age are similar to those of the Hyades cluster in Taurus.
Regulus, brightest star in the constellation Leo is six degrees of arc northeast of the moon on the night of April 13th.
The waxing gibbous moon is in the constellation of Virgo on Saturday April 16th, it along with Saturn and the first magnitude star Spica form a triangular pattern. At 8 p.m. the moon is eight degrees from Saturn and 13 degrees from Spica.
If you didn’t see the “Super” Full Moon of March 19th, don’t despair. Full Moons often look wonderful, especially when they’re not far above the horizon after rising and before setting. If the sky is clear, this month’s full lunar phase becomes visible above eastern horizons of New York City as it rises at 7:32 p.m. Sunday evening April 17th. Traditionally known by names such as “the Full Pink Moon” and “the Sprouting Grass Full Moon”, the Full Moon this month is only slightly further from us than on March 19th when it looked about one percent wider and 1.5 percent larger in area.
At six a.m. Saturday April 30th, a narrow crescent moon may be seen above an interesting arrangement of planets including Venus, Mercury, Jupiter, and Mars. For observers using a telescope, the faint sixth magnitude planet Uranus might also be seen.
Rising Full Moons are usually wonderful sights. Between perigee and apogee, when the moon is closest and most distant from Earth, the apparent diameter of the lunar disk varies by only about 13 percent. Eye-catching Full Moons have historically been attributed to several causes including atmospheric refraction and mental interpretations. However, differential bending of light rays from the upper and lower parts of the rising lunar disk would actually make the moon look smaller from top to bottom. There is no wide consensus as to why we perceive the moon as looking larger when it’s slightly above the horizon compared to when it’s higher up in open sky. One intuitive explanation is that we often see the rising Full Moon next to things on the ground we know to be large. When these considerable objects are seen from a distance they can look similar in size to the moon, therefore by comparison the moon also seems to be quite large.
Among Full Moons I’ve enjoyed several are especially remembered, including one rising next to the Empire State Building when viewed from the western end of 34th street. At that distance the big building looked small and the moon by comparison looked big. You might try an experiment with a ping pong ball seen from a distance of 13.5 ft. The ball then will appear the same size as last month’s now famous Full Moon. Next, move your head back about an inch and a half. With that increased distance, the ball will look the size as this month’s Full Moon rising on April 17th. Does the ball actually look different when viewed from these two slightly different distances?
Full moons and special sky events such as eclipses and close planetary conjunctions can inspire life long memories. Astronomer Johannes Kepler is said to have fallen in love with the night sky as a boy when his mother took him outside the walls of their town to see a lunar eclipse. At a restaurant appropriately named the Moon Palace, I once heard a Nobel Laureate in physics reminisce he became fascinated with nature when as a very young child he was astonished one evening by the sight of an apparently huge Full Moon seen between buildings at the end of his Manhattan street.