|May 1, 2008|
Mercury — the closest planet to the Sun — makes its best evening appearance of the year this month. Mercury looks about as bright as it ever does, with the view made picturesque May 2 by its location just 2° south of the Pleiades star cluster (M45), which lies in the constellation Taurus the Bull.
At its brightest in May, the innermost planet shines at magnitude –0.9. Brighter than any star through May 8, Mercury makes a stunning sight above the west-northwestern horizon. A 2-day-old crescent Moon joins the scene May 6, when it lies less than 3° to Mercury's upper right. You'll need clear skies and sharp vision to detect the Moon. Its slender crescent will be only 4 percent illuminated.
For the best views, use binoculars just as twilight falls. Camera owners might want to try shooting some images. The most pleasing contain foreground objects — trees, a water tower, or a windmill, for instance — which enhance the scene. They form great silhouettes and add to an image's impact.
A telescope easily reveals Mercury's phase, which shrinks to half-lit by May 8. It then stands 8° north of Aldebaran, Taurus' brightest star. Although Aldebaran is a 1st-magnitude star, it nevertheless shines a full magnitude fainter than Mercury.
Astronomy Senior Editor Michael E. Bakich explains what's happening: "When Mercury is as far east of the Sun as it can get (called greatest eastern elongation), we see it as an evening star low in the west," he said. "When it's west of the Sun, we view it as a morning star in the east before sunrise."
Some elongations are better than others because of Earth's tilt and the stretched-out nature of Mercury's orbit. Even at its farthest from the Sun, Mercury appears no more than 28º away.
Mercury reaches its greatest elongation May 13/14, when it lies 22° east of the Sun and sets 2 hours after our star. Still located in Taurus, Mercury then reveals a 37-percent-illuminated disk through a telescope. If you've followed it from the start of May, you'll notice the planet now shines more than a magnitude fainter. Still, it outshines all but two current nighttime stars.
Mercury continues to dim all month. By the 18th, it equals Aldebaran in brightness, and it appears only half that bright by the 22nd. Its angular distance from the Sun declines as well, making the planet harder to see after this date.
Three planets brighten June nights
Finding earthlike worlds
|Mars and Saturn adorn June's western evening sky. Mars, a relatively tiny nearby world, contrasts nicely with distant, giant Saturn and its wonderful rings. The king of the planets, Jupiter, rises during early evening and dominates the sky all night, although it maintains an unusually low altitude.|
The solar system's two other giants, Uranus and Neptune, are fine binocular targets. You can catch them easily despite the month's short nights, which limit dark-sky observing.
Six months after Mars' December 2007 opposition, Earth has left the Red Planet far behind. If we could view the solar system from above Earth's orbital plane, we'd find our planet leads Mars by about 90°.
While diminutive in its telescopic appearance, Mars displays a nice orange hue that provides a pleasing contrast with bluish-white Regulus, the brightest star in Leo the Lion.
Mars starts the month in Cancer the Crab and crosses into Leo June 10. Track the Red Planet's nightly motion. You'll see Mars close within 43' of Regulus by June 30. Mars is slightly fainter than the 1.4-magnitude star.
Less than 5° east of the pair stands Saturn, adding its yellowish hue to the darkening summer sky.
The colors of planets and stars are subtle. They become most obvious when two objects of differing hues lie close together in the sky. That's exactly what observers get in late June, as Mars, Regulus, and Saturn gather together.
Delicate hue changes occur during sunset and dusk; in fact, colors change every minute after sunset. Watch the sky closely as darkness descends and this star-and-planet trio emerges into view.
A crescent Moon wanders 1.8° south of Mars June 7. By the next night, the Moon has crossed a 14° gap to reach a point 2° south of Regulus.
Seeing Saturn in a telescope is a highlight of summer viewing. Rapid rotation bulges the planet's equator outward, which gives Saturn's disk an obvious flattened look. Saturn spans 17" in its equatorial diameter, but 15" pole to pole.
Saturn's atmospheric features are notoriously subtle. Observing them requires special filters to pull out detail. Occasionally, an obvious white spot appears on the planet's wan disk. This signifies the rare development of a major storm system. In 1990, the then newly launched Hubble Space Telescope imaged the last prominent white spot. In 2006, though, observers recorded less-pronounced spots.
What's happening? Earth will pass through Saturn's ring plane in 2009. This will be the first time in 13 years Earth has made such a crossing. The rings are so thin, they disappear from view in anything other than a large telescope.
Saturn's largest moon, Titan, is easy to spot in small telescopes; dimmer moons represent more of a challenge, but they're easy in scopes with apertures of 8 or more inches. Titan passes closest to Saturn when it passes north of the planet, as on June 13 and 29. Look for Titan south of Saturn June 5 and 21.
Iapetus, the peculiar moon that appears brightest when farthest west of Saturn, fades throughout June. It lies north of the rings June 5 and 6, but at magnitude 11 it's tricky to spot. Iapetus reaches eastern elongation June 27 and dims by nearly a full magnitude (to 11.9).
Jupiter reaches opposition in early July, so it's visible almost all night this month. For the best telescopic views, observe the planet around its culmination in the south, which places it at least 20° above the horizon. Much lower, and the turbulence in Earth's atmosphere blurs the disk's fine detail.
Jupiter's relatively large disk spans 47" by month's end, more than double Saturn's apparent diameter. Jupiter's atmospheric details show much higher contrast than Saturn's, which makes for more interesting observations. Dark features called belts appear most obvious.
The darkest of these are the North and South Equatorial Belts that straddle the planet's midsection. Last year, a jovian weather event disrupted the South Equatorial Belt. This produced changes telescopic ob-servers could track nightly.
Jupiter's equatorial region rotates in 9 hours 50 minutes. Its northern and southern temperate regions take 5 minutes longer, which results in a 250-mph (400 km/h) difference in wind speed where the two regions meet. This boundary generates many of the eddies, spots, and festoons Jupiter is so well known for.
With practice, observers can detect changes in the location of these features within 5 or 10 minutes. The largest of the disturbances, the Great Red Spot, typically is visible every other night from any location.
Io, Europa, Ganymede, and Callisto, the four so-called Galilean moons, are visible in any telescope as they wander east and west of Jupiter's disk. Occasionally, these moons pass in front of or behind the planet. A moon's shadow is one of the darkest features observers can see on Jupiter. Just a few minutes of viewing will reveal the shadow's motion. Sequential digital images of such shadow transits create interesting time-lapse movies.
Pluto reaches opposition June 20. The dwarf planet lies near 6 Sagittarii, a magnitude 6.2 star that makes an easy guide to Pluto's field. The star lies 2° north-northeast of the fine binocular open cluster M23 (NGC 6494). On June 1, Pluto lies 0.2° northwest of 6 Sagittarii. Pluto's retrograde motion carries it farther westward, and by month's end the dwarf planet is almost a full degree away from the star.
Pluto glows at magnitude 13.9. It remains the only one of hundreds of Kuiper Belt objects in similar orbits that's visible using amateur telescopes. Even so, detecting Pluto visually is a challenge, requiring perfectly transparent skies, a high-quality 8-inch scope — and preferably one 10 inches or larger — and a detailed star chart. You can find such charts in The Observer's Handbook of the Royal Astronomical Society of Canada or Guy Ottewell's Astronomical Calendar.
Neptune is 2 months from opposition. It's a binocular object shining at magnitude 7.9, and it's easy to locate north of Delta (δ) Capricorni, the easternmost star in Capricornus. Neptune forms a triangle with two stars, 42 Capricorni (magnitude 5.2) and 44 Capricorni (5.9). Neptune's disk spans 2.3". The planet rises before midnight by June 30.
Uranus is now a morning object. It stands high in the south a couple of hours before dawn. It lies in Aquarius, standing 5° east of Phi (φ) Aquarii at the eastern side of the constellation. The "circlet" of stars in Pisces lies to its north. Uranus shines at magnitude 5.8, an easy binocular target. Telescopes will reveal its greenish-blue tinged disk spanning 3.5".
Mercury reappears in the morning sky after its June 7 inferior conjunction with the Sun. Look for it popping out of the morning twilight during the month's last few days.
On June 28, 0.7-magnitude Mercury rivals Aldebaran, which shines at magnitude 0.8. Mercury lies 7° to the lower left of Aldebaran, low in the brightening eastern sky. Begin watching an hour before sunrise. A waning crescent Moon stands watch above the pair. Mercury brightens to magnitude 0.6 by the next morning, when the Moon has moved east of the Pleiades (M45) and lies 12° above Aldebaran. The planet's visibility improves in early July.
This is the month when the Sun reaches its northernmost declination along the ecliptic, which means nighttime hours reach their minimum span. The event, called the summer solstice, occurs June 20 at 7:59 P.M. Eastern Daylight Time.