April 2012 Sky Calendar
APRIL 2012 SKY CALENDAR
Now, then, we promise to focus on April in the April sky calendar. One might consider this assurance peculiar, for Apri should naturally be our priority during this month of April. And, indeed, it should be. However, just as we were in summer 2003 when we prepared for the Mars close approach, we're so keen about the Venus transit that we risk treating all previous events as so much preface. (The Transit of Venus* happens June 5th for us!)
So..no preface this month! Well, except for this preface, but that hardly counts.
APRIL 1: THE WAXING GIBBOUS MOON 5.8 DEGREES SSW OF BEEHIVE STAR CLUSTER
Also called Prasepe, the Beehive Star Cluster is an open, or galactic, cluster within Cancer the Crab. In fact, Prasepe is the faint constellation's only prominent feature. Unfortunately, as the gibbous moon is quite bright, the lunar light will diminish our view of the cluster.
APRIL 3: MERCURY STATIONARY
Stationary planet events require an explanation, as the phrase, itself, is misleading. Planets never stop in their orbits. They, like everything else in this restless cosmos, always move. There lies the nub, since Earth, our observation platform, also moves. Watching other planets on this mobile world complicates the observation and makes other planets appear to move in impossible ways. Mercury appears to stop its progression through the sky, but, again, this is merely an illusion.
APRIL 3: VENUS NEAR ALCYONE (PICK EVENT!)
Alcyone is the brightest star within the Pleiades star cluster,poised atop Taurus the Bull. Seeing the love goddess gathered with Atlas' daughters makes for lovely viewing, especially since Venus is so bright. Because this juxtaposition of world and stars is as mythologically significant as it is visually pleasing, we've designated this gathering as the pick event! See the sisters and their visiting goddess in the western evening sky.
APRIL 3: MARS 8.3 SSW OF MARS
Well, April 3rd is turning out to be quite a day, isn't it? See Mars and the gibbous Moon in the eastern evening and western morning sky. As Mars is a disc light source, the Moon won't obscure it.
APRIL 6: FULL MOON
The Easter Moon, as it is called since it is the full moon immediately preceding Easter. The full moon is always a brilliant sight, even if it does wash out a lot of the sky.
APRIL 7: MOON 6.0 SSW OF SATURN
The advantage of these moon-planet appulses is that they enable observers to readily identify different planets On April 3, one can quickly find Mars by first finding the Moon. Tonight, Saturn is a cinch sight because of its proximity to the moon.
APRIL 8: EASTER
Easter is a Christian and an astronomical event, as the date commemorates Jesus' resurrection is astronomically determined. Easter is the Sunday after the first full moon that occurs on or after the Vernal Equinox. The Councile of Nicaea in AD 325 determined that the Easter date would be set according to this formula.
APRIL 13: LAST QUARTER MOON
The last quarter moon rises around midnight and sets around noontime. Yes, one can see the Moon as often during the day as one can at night. Of course, at night, the Moon doesn't have to compete with the Sun for your attention.
APRIL 15: MARS STATIONARY. (Hey, let's just copy and paste most of the text we wrote for the paragraph about Mercury being stationary.)
Stationary planet events require an explanation, as the phrase, itself, is misleading. Planets never stop in their orbits. They, like everything else in this restless cosmos, always move. There lies the nub, since Earth, our observation platform, also moves. Watching other planets on this mobile world complicates the observation and makes other planets appear to move in impossible ways. Mars appears to stop its progression through the sky, but, again, this is merely an illusion.
APRIL 15: SATURN AT OPPOSITION
Envision the solar system from "above," whatever the devil that means in outer space. On April 15th, Earth passes between the Sun and Satun, so that the three bodies form a nifty straight line. Saturn will therefore rise around sunset and set around sunrise, just like the full moon.
APRIL 18: SUN ENTERS ARIES
Throughout the year, the Sun appears to travel through thirteen constellations, the patterns comprising the zodiac. These constellations are: Pisces the Fish, Aries the Ram, Taurus the Bull, Gemini the Twins, Cancer the Crab, Leo the Lion, Virgo the Maiden, Libra the Scales, Scorpious the Scorpion, Ophiuchus the Serpent Charmer, Sagittarius the Archer, Capricornus the Seagoat, and Aquarius the Waterbearer.
APRIL 18: MERCURY AT GREATEST WESTERN ELONGATION (27.5 degrees)
Elusive little Mercury reaches its greatest western elongation. When at western elongation, an inferior planet will be in the eastern pre-dawn sky. This is a splendid time to look for Mercury. (See Planet Watch)
APRIL 18: MOON 7.0 DEGREES NNW OF MERCURY
April 18th is another busy day. Find the waning crescent moon close to Mercury in the eastern pre-dawn sky. Mercury is often difficult to find even at greatest elongation. Today, however, you can use the Moon to help you spot the first world.
APRIL 21: NEW MOON
The new moon is not visible, unless, of course, it eclipses the Sun. No eclipse this month, however. Lunation cycle 1105 begins.
APRIL 22: LYRID METEOR SHOWER PEAKS
Now, why not select this meteor shower peak as the month's pick event? Well, we have to let you in on a deep secret: we don't trust meteor showers anymore than we can throw them. Astronomers predict that this Lyrid peak will be "very favorable," meaning that observers should see quite a few meteors. One reason astronomers are so optimistic is that the Moon, which will be in the first day of its new cycle, won't interfere. Each meteor shower is assigned a ZHR number: ZHR meaning Zenithal Hourly Rate. This value is equal to the number of meteors a seasoned observer would see if he/she were in a completely dark sky and the radiant were at the zenith. (Radiant: the apparent origin point of meteors; zenith: the point directly overhead.) Since the radiant is along the border between Lyra (hence the name) and Hercules, it will be quite close to our zenith around 4:00 a.m. However, think of the ZHR as being similar to the gross amount on a paycheck. Subtract a series of deduction and one will have the actual rate. Perhaps, if you venture out in the late evening and look toward the northeastern sky, you might see 20 - 45 meteors an hour.
APRIL 22: MOON 2.5 DEGREES N OF JUPITER
Hit or miss. The thin crescent moon and Jupiter will be about 15 degrees from the Sun. You might see it if you look within half an hour of sunset. A difficult sight.
APRIL 24: MOON 5.7 DEGREES S OF VENUS
A much easier sight. The Moon will be a couple days older, while Venus will be brighter than Jupiter and higher in the sky. Seeing the Moon and Venus together is always a treat for the eye
APRIL 29: FIRST QUARTER MOON
APRIL 30: VENUS AT MAXIMUM BRIGHTNESS FOR THE YEAR
Planet brightnesses vary because their distances from both the Sun and Earth constantly change. A planet's brightness depends on four factors: its albedo (the percentage of sunlight it reflects); its size (which determines the surface area from which it reflects the light); its distance from the Sun (the closer it is to the Sun the more intense the incidental light on it will be) and its distance from Earth (the inverse square law tells us that a light source's brightness diminishes with the square of the distance.) At magnitude -4.5, Venus is about 16 times brighter than Sirius, the night sky's brightest star.
MERCURY: Best seen late month. The first planet reaches greatest western elongation (27.5 degrees) on April 18th. As mentioned above, an inferior planet is in the eastern morning sky when at greatest western elongation. VERDICT: Go out late month to see Mercury in the pre-dawn sky.
VENUS: Well, what can we say? Always brilliant, Venus grows all the brighter throughout the month as it veers closer to the horizon at sunset. The sister sphere is close to transit and throughout April we watch its evening descent toward the Sun. VERDICT: Bright, but low? No problem if you venture out soon after sunset to behold this splendid spectacle.
MARS: The War God world's brightness diminishes throughout the month. At month's beginning, it will be brighter than any night sky star except Sirius. By month's end, it will be as bright as Arcturus: the fourth brightest night sky star. Of course, Mars will still be bright by the end of April. Still within Leo the Lion, Mars starts the night in the eastern sky and sets in the wee hours. VERDICT: Still an easy sight, particularly because of its reddish hue. Find Mars in the evening eastern or morning western sky.
JUPITER: The great bloated one is quite low in the western evening sky. On May 13th, Jupiter will be in solar conjunction, so May will hardly be Jupiter's month. In April, we watch it descending lower toward the horizon by sunset. VERDICT: If, like Selene, you just HAVE to see Jupiter in his unattired glory, best to venture out the first half of April. Jupiter remains brighter than Mars and Saturn, but not in great viewing position. Jupiter will return to the pre-dawn June sky.
SATURN: (PICK PLANET!) Saturn gets the coveted DA nod because it reaches opposition on tax day. When at opposition, the planet rises around sunset and sets around sunrise. By mid-month, Saturn is as bright as Mercury, but won't outshine Mars until May. VERDICT: We didn't confer the "pick planet" honor on it just because we like it. Saturn is easy to find this month as it is literally up all night.
*A transit of Venus is the direct passage of Venus across the Sun.