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Share 04/2/2012 April Skies by Dick Cookman Highlights: Comet Journal, Martian Landers, Meteor Showers, Analemma, Planet Plotting, April Moon Focus Constellations: Cassiopeia, Cepheus, Ursa Minor, Draco, Bootes, Ursa Major, Lynx, Leo, Cancer, Gemini, Auriga, Camelopardalis, Perseus Comet Journal Comet C/2009 P1 (Garradd) is at 7th magnitude and west of Dubhe in Ursa Major. It is in the northern sky near the meridian after sunset and can be found throughout the night. It will move into Lynx at the end of April. It is above the orbit of Mars and on its long journey to the outer solar system and will dim to 8th magnitude in May and 10th or 11th magnitude in June. Comet 2011 F1 (Linear) is quite dim at 12th magnitude and is currently between Draco and Bootes. It will brighten as it approaches the Sun and Earth and may reach 9th magnitude when it is closest in the autumn. Comet 96P/Machholz 1 (2012) is in southern hemisphere skies and may reach 7th magnitude as it moves into the skies of the northern hemisphere and passes Earth in July. Comet 21/P Giacobini-Zinner has decreased to 12th magnitude and sets in the west right after the Sun. Martian Landers Opportunity is on the western rim of Endeavor Crater at a site called Greeley Haven on the north end of Cape York where it has a northward tilt of 15 degrees for favorable solar energy production. Even though the winter solstice for Opportunity was on March 29th, the expected decrease in solar electricity production was avoided due to a slight cleaning of the solar panels by Martian winds. Solar array production on Sol 2879 (Feb. 28, 2012) was actually 10% higher that that produced on Sol 2872 (Feb. 21, 2012). The rover is conducting radio Doppler tracking measurements which will enable scientists to determine small variations in the rotation and nutation (wobble of rotational axis) of Mars. These are influenced by variations in the interior of the planet and may allow more detailed understanding of the nature of the core and mantle of Mars. Fifteen Radio Doppler tracking passes were performed from Sol 2870 (Feb. 19, 2012) and Sol 2903 (Mar. 25, 2012). Further studies of the nearby rock target (Amboy) were conducted during the interval with the microscopic imager and the Mössbauer spectrometer and imaging with the Panoramic Camera (Pancam) revealed additional details in the nature of the surrounding terrain. Meteor Showers The Lyrid Meteor shower on the night of April 20/21 coincides with New Moon providing the opportunity to observe up to 25 meteors per hour. The 1982 shower produced a maximum of 90 meteors per hour in dark skies. Lunar glare will not be a factor and Lyra rises in the northeast about 9PM EDT so conditions should be close to ideal. Peak rates should occur during the predawn hours when North America is on the leading edge of the Earth as it plunges into the midst of the stream of debris left in the wake of Comet Thatcher (C/ 1861 G1). The comet was discovered in 1861 when it last visited the inner solar system and it will not return to recharge the meteor stream until 2276. Analemma The Analemma is a graph plotting changes in the position of the Sun through the year. I saw a photographic analemma in Sky & Telescope Magazine a few years ago which was produced by making multiple exposures of the Sun through the year at the same time of day without advancing the film. The analemma can be used to produce the graph which looks like a figure 8 which is often found on maps and Earth globes. It is a plot of the variance of apparent solar time (sundial time) derived from the position of the Sun in the sky and mean solar time. The latter averages out this variance over the year. The two vary due to the changing rate of the Sun's apparent movement through the sky which is influenced by two factors: A. The first is Earth's revolution in orbit. The orbit is elliptical with Earth moving fastest at perihelion (near Sun) in January and slowest at aphelion (largest solar distance) in July. As a result: Earth must rotate farther each day near perihelion for the Sun to appear at the meridian so the Sun appears to be moving more slowly than average. B. The second is that the tilt of the axis of the Earth contributes to the apparent speed of the Sun through the sky in that the Sun's motion is at a maximum at the equinoxes when the Earth's equator is directly below the Sun and all of the Suns' motion is along the equator. It is at a minimum at the solstices when the Sun appears above the tropics of Cancer and Capricorn at its farthest distance from the equator. At this time the major component of the motion of the Sun through the sky is at an angle to the equator so solar motion relative to the equator is at a minimum and the Sun is slow. Mean solar time is measured at the equator and combination of the two motions produces the slowest Sun on February 11th when solar noon is 12:14.25 PM EST and the fastest Sun on November 2nd when solar noon is at 11:44.52 AM EST. The Sun is on time on April 15th, June 12th, September 1st, and Dec. 24th when solar noon occurs at noon. The Sun reaches a slightly fast peak on May 13 and a slightly slow peak on July 25th. Planet Plotting Venus, Mars, Jupiter, and Saturn are evening planets in April. Jupiter(-2.1) in Aries sets about one hour after sunset and Venus (-4.7) in Taurus follows after 11PM EDT. On the 30th, two days after Astronomy Day, Venus achieves greatest brilliance of the year at magnitude -4.7. Mars (-0.4) in Leo is in the southeast and south in the evening and sets about 4AM EDT while Saturn (+0.3) in Virgo, at opposition on the 15th, is at its brightest in 2012 as it rises at sunset and sets at sunrise. Mars in Leo moved through opposition in March and is decreasing in magnitude as Earth speeds away. It drops from magnitude -0.7 to -0.1 in April and will be over 5 times dimmer by the end of 2012. Mercury and Uranus (+5.9) in Pisces are morning planets which rise less than an hour before sunrise in April. They are within 2° of one another on the 21st but are hard to find in the glow of sunrise. Mercury (+0.2) reaches greatest eastern elongation on the 18th. Neptune (+8.0) in Aquarius is another morning planet which rises slightly more than an hour before the Sun in April. Planet Constellation Magnitude Planet Passages Sun Pisces, Aries -26.8   Mercury Pisces +2.1 to -0.1 Uranus, 2.0°N, 4/21, 10PM EDT Greatest western elongation, 4/18, 1PM EDT Venus Taurus -4.5 to -4.7 Greatest brilliance, 4/30, 4AM EDT Mars Leo -0.7 to -0.1   Jupiter Aries -2.1 to -2.0   Saturn Virgo +0.3 Opposition, 4/15, 2PM EDT Uranus Pisces +5.9 Mercury, 2.0°S, 4/21, 10PM EDT Neptune Aquarius +7.9   April Moon The New Moon of March 22nd at 10:37 AM EDT marked the start of lunation #1104 which will end 29.19 days later with the New Moon on April 21st at 3:18 AM EDT. The Full Moon of lunation 1104 is in Leo at 3:19PM EDT on the 6th. April's Full Moon is the "Egg, Grass, Easter or Paschal Moon". To colonial Americans it was the "Planter's Moon" and medieval English called it the "Seed Moon". The Celts referred to it as the "Growing Moon" and the Chinese thought of it as the "Peony Moon". Anishnaabe (Chippewa and Ojibwe) of northern Michigan call it “Iskigamizige-giizis(oog)" (Broken Snowshoe Moon). On April 7th at 1PM EDT, the Moon will be closest to Earth at perigee (222,544 miles or 56.18 Earth radii). Lunar apogee is on April 22nd at 10AM EDT when it will be farthest from Earth at 252,411 miles or 63.72 Earth radii. Planet Constellation Magnitude Moon Passage Moon Phase/Age Sun Pisces -26.8 3:18AM EDT. 4/21 New ~ 0 days Mercury Pisces +0.3 8.0°S, 10PM EDT, 4/18 Waning Crescent ~ 26.98 days Venus Aries -4.7 6.0°N, 10PM EDT, 4/24 Waxing Crescent ~ 1.78 days Mars Leo -0.7 9.0°N, 11PM EDT, 4/3 Waxing Gibbous ~ 12.02 days Jupiter Aries -2.0 2.0°S, 3PM EDT, 4/22 Waxing Crescent ~ 1.03 days Saturn Virgo +0.3 6.0°N, 10AM EDT, 4/7 Waning Gibbous ~ 16.02 days Uranus Pisces +5.9 6.0°S, 4AM EDT, 4/19 Waning Crescent ~ 27.22 days 2.0°N, 10PM EDT, 4/21 Waxing Crescent ~ 0.03 days Neptune Aquarius +7.9 6.0°S, 11AM EDT, 4/16 Waning Crescent ~ 24.52 days