Sacred monuments

Near Keswick, in England’s Lake District, stand two stone circles dating to about 2000 B.C. A smaller circle consists of 10 stones and is surrounded by another circle of 28 stones. All but five still stand. The structure, called Castlerigg, has been shown to mark the points of seven solar and lunar alignments in much the same manner as Stonehenge.

The most startling aspect of Castlerigg was discovered in 1976 when a photographer prepared to capture the midsummer sunset. He noted that as the sun hovered just at the top of the ridge, its rays were blocked by the tallest stone in the outer circle, throwing a shadow for hundreds of feet down the slope behind the circle.

If this is not by pure accident, no one can hazard a guess as to how the ancients were able to manage a feat that should have required sophisticated, modern surveying instruments.

Focus on the planets

Four planets and two comets make May a banner month for observing the night sky.

Mercury makes its appearance in mid-May low in the east about an hour before sunrise. Look for Mercury on May 16 just to the lower left of the thin crescent moon.

Venus blazes up on the western horizon an hour after sunset as May opens. This represents its maximum brightness for the current appearance, then it will plunge into the twilight’s glare by the end of the month.

Mars may be found to the upper left of Venus at the beginning of the month. The Red Planet is now nearly on the opposite side of the sun from Earth and is a dim, fuzzy reminder of its past glories a few months ago.

Jupiter is visible high in the south shortly after sunset. It is second in brightness only to Venus and viewers with a telescope can observe the giant planet’s bands, the Great Red Spot and the parade of major moons across the planet’s face.

Saturn appears in May to the upper left of Mars an hour after sunset but switches places with the Red Planet by month’s end as all three planets dip towards the western horizon. Saturn is tilted to allow highly favorable viewing of its ring system.

Neptune and Uranus are high in the southeast during the hours before dawn. A map to locate them may be found in the April issue of Sky & Telescope.

May events

1 Sunrise, 5:26 a.m.; sunset, 7:40 p.m. Check the western sky an hour after sunset where Venus, Mars, and Saturn form an ascending leftward diagonal well up on the horizon. This is May Day, or Beltane, a cross-quarter day marking the mid-point between the spring equinox and summer solstice. Some mark the event on May Eve, or April 30.

4 Full moon, 4:34 p.m. The full moon of May is called the Planting Moon or Milk Moon. A total eclipse of the moon occurs in Europe, Africa and parts of Asia.

6 The moon is at perigee, or closest approach to the Earth, today.

13 The sun enters Taurus the Bull on the ecliptic.

19 New moon, 12:53 a.m.

20 The sun enters the astrological sign of Gemini, even though, astronomically, it has just entered Taurus.

21 The moon is at apogee, or greatest distance from the Earth, today.

22 Saturn and Mars are nearly side by side on the western horizon tonight, with the crescent moon directly above them. The two stars directly above the moon are Castor and Pollux of Gemini, while Betelgeuse hovers just above the horizon far below.

27 Moon in first quarter, 3:57 a.m.

30 Whitsun, the 7th Sunday after Easter.

31 Sunrise, 4:53 a.m.; sunset, 8:13 p.m. Take this last opportunity to check out the three naked-eye planets in the west after sunset. Venus shines just above the horizon with Saturn to the upper left. Mars is just above Saturn with Castor and Pollux higher still.

Comet alert!

Two comets will grace the sky in May. One, NEAT, will be visible in the southwest near Procyon of Canis Minor on May 9. It will continue northeast, passing by the Beehive Cluster around May 15, and ending the month in the Big Dipper. NEAT may be visible by naked eye but good binoculars or a telescope should bring the fuzzy comet into clear view. The other, LINEAR, will make two brief appearances for Northern Hemisphere viewers. The first week of May finds it peeking above the eastern horizon at dawn, however, it likely will be lost in the solar glare. Late in the month, it will be very low in the southwest among the stars of Hydra.

Clair Wood taught physics and chemistry at Eastern Maine Technical College in Bangor.