On a frigid shore
A week after New Moon, the Sun rises over the lunar crater Aristoteles. This 54-mile-wide crater lies on the southern shore of Mare Frigoris, in the Moon’s northeastern quadrant. It forms a beautiful pair with Eudoxus to its south, near the long chain of the Caucasus Mountains. As sunlight illuminates the crater’s inner walls, elaborate terracing becomes noticeable. Hills dot the crater’s floor, and the highest peak lies off-center, near the southern wall.
The northwestern wall appears particularly bright as the first rays of sunlight strike it, and it shows evidence of slumping – the floor at the wall’s base is very hilly. The crater’s rim rises 2 miles above the floor. Tiny craterlets pit the outer walls, and a small crater, Mitchell, makes an impression just east of Aristoteles. Enjoy the rapidly changing appearance of this magnificent crater in the days surrounding First Quarter phase. Martin Ratcliffe is former president of the International Planetarium Society. Alister Ling is a meteorologist for Environment Canada.
A partial eclipse can be seen across southern North America and western South America. Use the map at right to find out how much of the Sun the Moon will block. As always, any time you observe the Sun when it is not totally eclipsed, use proper eye protection.
The month’s second eclipse occurs the morning of April 24. This is a penumbral lunar eclipse, which means the Moon passes through the outer, lighter part of Earth’s shadow. Observers with sharp eyes in most of North and South America should see the northern half of the Moon take on a slightly darker hue as our satellite sets that morning.
Harping on meteors
April’s claim to meteor fame is the annual Lyrid shower. This stalwart peaks this year the night of April 21/22. Unfortunately, Full Moon arrives just 2 days later, and its brilliance in the night sky will significantly reduce the number of visible meteors. Your best bet is to observe before dawn the morning of April 22, when you might see up to 10 meteors per hour. Lyrid meteors appear to radiate from the constellation Lyra the Lyre, which climbs highest in the predawn hours. Comets and asteroids
All night long
Occasionally, a comet rewards us by staying visible throughout the night. In March 1996, the great comet Hyakutake passed near Polaris and remained on view all night. This month, Comet C/2004 Q2 (Machholz) fits the bill. Meteors and moons
An American eclipse
The rarest type of solar eclipse returns to Earth this month after 18 years. Just 5 percent of all solar eclipses are hybrid: total over part of the path, and annular over the rest. On the section of the eclipse track near local noon, that part of Earth lies close enough to the Moon for the eclipse to appear total. But at the ends of the track, around sunrise and sunset, those parts of Earth lie far enough from the Moon that our satellite can’t cover the whole Sun. Observers there see an annular eclipse, a thin ring of Sun left around the Moon. The Moon’s shadow sweeps eastward across our planet April 8, bringing a rare hybrid solar eclipse to Earth’s surface. Along a narrow swath of the Pacific Ocean, the Moon completely covers the Sun for up to 42 seconds. Along a similarly constricted path through Panama, Colombia, and Venezuela, a spectacular “Ring of Fire” will impress observers.
Elsewhere across southern North America and western South America, the Sun appears partially eclipsed. The Moon’s shadow makes landfall on North America’s west coast in mid-afternoon and leaves Earth off the Atlantic coast at sunset. Amateur astronomers will be out in force to show the eclipse to the general public – ask at the nearest planetarium or science center.
As always, exercise caution when viewing a partial solar eclipse. Never look directly at the Sun with your eyes or through binoculars or a telescope without a proper solar filter. You risk partial or total blindness. You can purchase a safe filter from one of the companies that advertises in Astronomy or at your local science center.
Jupiter continues to impress observers with its display of cloud belts, white ovals, and festoons. The biggest planet in the solar system reaches an important milestone – opposition – April 3. Jupiter then lies opposite the Sun in our sky, rising near sunset and setting near sunrise. For the best views through a telescope, wait until mid-evening for Jupiter to climb above the turbulence in the lower levels of our atmosphere. All four Galilean moons line up on one side of Jupiter the evenings of March 31, April 7, and April 21.
As wonderful as the views of Saturn sent to us by the Cassini spacecraft are, seeing Saturn with your own eyes through a telescope can be more rewarding. Take a close look at the shadow of Saturn’s globe falling on the rings behind the planet. Week by week during April, the shadow appears to grow because Earth’s orbit carries us a little to the side, giving us a slightly different perspective on the ringed world.
Mars lingers in the morning sky, where it appears fairly inconspicuous to the naked eye and totally unimpressive through a telescope. Better views await observers this summer and autumn.
Comet C/2004 Q2 (Machholz) remains a lovely sight in binoculars from a dark-sky site. It stays visible all night, circling the pole as it steps over the Dragon’s tail and enters the den of the Great Bear. Away from city lights, the realm of galaxies in Virgo beckons. As if trying to show the way, asteroid Pallas glows just one or two fields away from several galaxies. It is well within reach of the smallest telescope.
A second eclipse occurs this month, but it will be subtle. The Moon passes through Earth’s lighter shadow, called the penumbra, the morning of April 24. Sharp-eyed observers will see the Moon’s northern half take on a yellowish shade as it sets near sunrise.
As it traverses the sky, the Moon creates other exceptional events. It sneaks into the western evening sky as a hard-to-see, day-old crescent April 9. The following night, it appears as a gorgeous bright arc, with the dark side filled with blue-gray earthshine. Don’t miss the splendid binocular sight when the Moon pairs up with the Pleiades star cluster April 11. Cresting above Saturn on the 15th, the Moon then covers up Jupiter for people in southern Africa a week later. Europeans see a similar occultation of the red supergiant star Antares the morning of April 27. Mars rises around 3 a.m. April 1 and an hour earlier by month’s end. During this period, it crosses from northern Capricornus into Aquarius. You can follow its eastward progress all month. On April 8, Mars lies 0.7° south of Theta (θ) Capricorni; on the 14th, 0.2° east of Iota (ι) Cap; on the 20th, 1.2° north of Gamma (γ) Cap; and on the 22nd, 1.1° north of Delta (δ) Cap. Finally, the planet passes just one Moon-width (0.5°) north of the 4.3-magnitude bluish star Iota Aquarii April 29. Shining at 1st magnitude, Mars appears conspicuously brighter than any of these stars.
As our planet’s orbital motion brings us slowly closer to Mars, the Red Planet grows slightly in apparent diameter. It reaches almost 7″ across by the end of April. Experienced digital imagers should be able to pull out some detail on the martian surface now, but most of us will have to wait until midsummer before the planet becomes a decent target for our telescopes. Mars will reach a favorable opposition in early November.
On April 12/13, Mars passes 1.2° south of Neptune. Although Neptune is intrinsically a much larger planet, its greater distance makes it appear small and faint. Neptune currently lies some 20 times farther from Earth than does Mars. At this conjunction, Mars shines at magnitude 0.8, while Neptune glows at magnitude 7.9, nearly 1,000 times fainter. You’ll need binoculars to spot Neptune. Neptune’s disk measures 2.2″ across and requires a small telescope and good atmospheric seeing conditions to see it clearly.
Uranus reappears in the morning sky at the end of April. On the 30th, it lies 1.7° southwest of 4th-magnitude Lambda (λ) Aquarii. Mars is the most prominent nearby object, located 10° to the upper right of Uranus. You’ll want to use binoculars to spot Uranus, which glows at magnitude 5.9. Look for it several degrees above the east-southeastern horizon at least 75 minutes before local sunrise.
Innermost Mercury has its poorest morning apparition of the year for Northern Hemisphere viewers this month – it never climbs more than a few degrees above the eastern horizon before twilight makes it impossible to see. Mercury reaches greatest western elongation April 26, when it shines at magnitude 0.4 and stands 4° high in the east 30 minutes before sunrise. The next favorable appearance of Mercury will be in the evening sky in early July. It currently resides in the far northern sky and does not set unless you live south of the United States or Europe. Shining nicely at about 7th magnitude, this dirty snowball is easiest to find under a dark sky. Try sweeping with binoculars about halfway between Polaris and the bowl of the Big Dipper. From a city backyard, you’ll probably need a bit more aperture. Use the stars Lambda and Kappa (κ) Draconis to start your star-hop.
A short dust tail should be pointing away from Polaris. Its apparent length depends on how dark your sky is. Observers in rural areas always see a longer tail than those who live near major sources of light pollution.
With a couple of notable exceptions, namely Andromeda’s M31 and Triangulum’s M33, Comet Machholz glows brighter than any galaxy in the Messier catalog. This means an 8-inch scope from the suburbs will reveal some inner detail. After a survey of the comet’s head and tail at low power, bump the magnification up to 120x or so.
Star charts of this area typically plot the large, eye-catching galaxy NGC 4236 and will list an appealing total brightness of magnitude 9.6. But beware: The galaxy’s light spreads out over such a large area that this object proves a considerable challenge in small scopes. The barred spiral has been glimpsed in 4-inch scopes under extremely dark skies.
Asteroid 2 Pallas appears bright because it is large (325 miles across) and comes relatively close to Earth. Heinrich Olbers discovered Pallas in March 1802, making it the second asteroid to be found. This month, Pallas travels in front of the “realm of the galaxies,” making it a roving guide through what many beginners consider a scary part of the sky.
A primary key for unlocking secrets of the Virgo cluster is the star 6 Comae Berenices. If you get lost, don’t worry. Just go back to the tail of Leo, hop along the base to 6 Comae, and start again. Between April 7 and 9, Pallas lies less than a Moon-width away from this 5th-magnitude star.After 6 Comae and four or five 6th- to 7th-magnitude stars, Pallas is the next brightest object in this area. From one night to the next, it should be easy to track this main-belt asteroid as it moves slowly across the sky. The deep sky
Any of the Deep Field images beamed back by the Hubble Space Telescope shows the fathomless depths of the universe. Only 80 years ago, Edwin Hubble himself was striving to convince other astronomers all those fuzzy elliptical and spiral clouds in Virgo were actually galaxies like the Milky Way, not small solar systems in development. His efforts helped transform the universe from a mind-blowing size of some 100,000 light-years to a stupefying vastness billions of light-years across.
Novice observers are justifiably a little scared of diving into what Hubble called “the realm of the galaxies.” Without the proper techniques and approach, panic can set in. To remain calm, first learn how your telescope shows the sky. How much sky will you see at low power? At 60x, depending on the eyepiece design, you’ll be looking at a field diameter of close to 1°, or approximately twice the apparent size of the Full Moon. Lower power makes it easier to get around. Next, you need to match what you see to the stars shown on the chart at right.
Now, which way is up? Newtonian and Dobsonian reflectors simply invert the image, so just turn the chart upside down. Schmidt-Cassegrain telescopes and refractors with a star diagonal flip the image left to right but keep north up. So you need to copy the chart onto a transparency and flip it over.
The last bit of confusion to conquer occurs when you point your scope at an object that does not lie due south. In this case, north and south on the sky do not lie straight up and down, but appear at an angle. Slowly turn the chart in your hands until its north points back to Polaris (for SCTs and refractors with a star diagonal), or until the chart’s south points to Polaris (for Newtonian and Dob owners). Now, your chart is oriented the same way as the eyepiece field. Practice this skill, and you won’t get lost again.
For your first foray into the galaxy realm, anchor yourself to the star 6 Comae Berenices. Then, shift one Moon-width west to hit M98, and slide back. Using the star pattern on the chart, step along the stars to the east and a bit north to the circular glow of the spiral galaxy M100. Next, plunge north past a field star to reach 11 Comae, which lies some 2°, or 4 Moon-widths, away.
If you have a drive on your telescope, turn it off for 5 minutes and relax while the sky slowly swings M85 into view. Backtrack to 11 Comae, south to M100, and west to 6 Comae.
Now you’re ready for the deep end of the Virgo cluster. Draw the field size of your telescope/eyepiece combination on the chart. Begin again at 6 Comae, but this time, head southeast (down in the early evening sky) 1.5 Moon-widths to find M99. With firm resolve, keep going in the same direction another 4 Moon-widths to land on M84 and M86, both roundish, fuzzy clouds. This pair marks the primary signpost of the Virgo cluster.
If you get lost, don’t get frustrated – simply reset your scope on 6 Comae and head back to M84 and M86. The biggest mental shift you face is that from here, you need to galaxy-hop instead of star-hop. Swim south and east to M87, and back again. Then, head northeast to locate M88 and M91 before returning to the M84/M86 pair.
With short out-and-back forays like this, you can remain calm enough to enjoy the experience. The first few times you’re in this area, tour only at low power to solidify your telescope skills. Then, go back for a longer look at higher power. The Planets
Two brilliant planets appear soon after sunset and provide a visual feast for every observer. Saturn stands high overhead as soon as darkness falls – it’s one of the first “stars” to come out. The second bright planet on view is Jupiter. Look for it in the direction opposite to where the Sun has set. Mars belongs to the morning sky, where it has a brief encounter with Neptune at midmonth.
Saturn, which starts high in the sky during twilight and sets after midnight, remains visible all evening among the stars of Gemini the Twins. A short distance above the planet lie the two brightest stars in Gemini, Castor and Pollux. At magnitude 0.1, Saturn shines noticeably brighter than this prominent pair. On April 15, a First Quarter Moon joins the trio, with the ringed planet lying 5° below the Moon. Saturn is now 3 months past opposition, so its shadow appears clearly on the rings east of the planet’s disk.
January saw high drama when the Huygens probe made the first-ever descent through the atmosphere of Saturn’s brightest moon, Titan. Many of your friends might be surprised to learn that you can spot this 8th-magnitude moon easily through a small telescope. This month, you’ll find Titan due west of Saturn April 8 and 24 and due east April 16. Titan passes due north of the planet April 4 (when a fainter moon, Iapetus, lies close by) and April 20. Titan appears due south of Saturn April 12 and 28.
Iapetus travels around Saturn in a larger orbit than Titan, and consequently, Iapetus moves more slowly. In early April, Titan and Iapetus lie in close proximity, but Titan pulls ahead quickly. Iapetus shines at 11th magnitude during the first few days of April and brightens slowly as it approaches western elongation April 24. By this date, Iapetus glows at its brightest, magnitude 10.1, comparable to Tethys, which orbits much closer to the planet. Iapetus then lies farthest from Saturn of any visible moon. Titan lies nearly 3′ west of Saturn, while Iapetus is a full 8′ west. Shining at magnitude –2.5, Jupiter maintains its brilliance throughout April. Its location opposite the Sun in our sky underlines the planet’s opposition that occurs April 3. When any outer planet achieves this special position, it remains visible all night. Opposition also brings the planet closest to Earth, so it shines brightest and appears largest, making this the best time for viewing. Furthermore, opposition places a planet due south at local midnight, the position where we view it through the least amount of Earth’s atmosphere.
At this year’s opposition, Jupiter’s apparent size in a telescope reaches 44″ – the largest of any outer planet. (Venus can get larger, but only in twilight and as a thin crescent.) This large apparent size offers great opportunities for viewing cloud features in the jovian atmosphere. Jupiter completes one rotation in less than 10 hours, allowing the entire planet to be viewed during a single night this month.
Make the most of Jupiter’s appearance this year because next year, it will lie in Libra and never climb much higher than 35° above the horizon for observers at mid-northern latitudes. This year, it lies in Virgo and stands nearly 50° above the southern horizon around midnight. Higher altitude also reduces the effects of atmospheric turbulence.
On April 3, Jupiter lies 10° northwest of Virgo’s brightest star, 1st-magnitude Spica. Throughout April, the planet’s westward motion carries it toward Porrima, Virgo’s second-brightest star, so that by the 30th, Jupiter lies 1.3° south of it. On April 22, you’ll find Jupiter 6° to the upper right of the almost Full Moon.
Jupiter’s four bright moons, Io, Europa, Ganymede, and Callisto, continue to perform their elegant dance around the planet. Io takes less than 2 days to orbit, so you can see its relative position change in an hour or so – less when it appears close to Jupiter. Our line of sight lies in the plane of the moons’ orbits, so we see occultations, eclipses, and transits of the moons at various times.
Moving from the night’s brightest planet to the faintest, Pluto lies near a 3.5-magnitude star in Serpens Cauda, a constellation that appears above Sagittarius in the southeastern morning sky. Early April finds Pluto 24′ north-northeast of Xi (ξ) Serpentis. Having a bright star near the planet’s location drastically cuts the time required to find the right field of view. By the end of April, Pluto’s apparent distance from Xi has grown 50 percent, with the planet moving to the northwest of the star from night to night. If you own an 8-inch or larger telescope, you should be able to reach stars as faint as 14th magnitude.
In that case, Pluto’s motion relative to the background stars will betray its identity. Although Pluto rises before midnight, don’t think of searching for it until after midnight, when the area reaches a decent altitude. The region lies due south at approximately 5 a.m. local daylight time.