Touring the summer sky.
By the end of dusk the sky was cloud-free and very clear, telescopic images were much steadier than expected, and invitations were flowing from us veterans in overlapping cascades: "Come look at Comet McNaught-Russell near a star . . . the Beehive cluster looks great in the 11x80 binoculars . . . these pairs of stars in Ursa Major are called the Three Leaps of the Gazelle . . . here's Theophilus, a superb lunar crater." And so on.
What about the moonlight? Didn't it spoil some of what we wanted to see? In springtime a Moon that sets near 1 a.m. can be as young as 6 days from new, as this one was -- not quite first quarter. Isn't that still too bright for observing faint objects? All I can say is that with the clear sky and country site, the diffuse comet looked mighty good in telescopes, and many stars of the Coma Cluster were easy with the naked eye.
The whole evening was a surprise, but after the novices left (heads happily swimming with information and visions) there was a finale: unexpected lunar occultations! We three veterans did a do-si-do dance of switching telescopes while each star approached the Moon, then we froze at our telescopes of choice as each star hung on the dark lunar limb and suddenly winked out, to nearly simultaneous yells and whoops of delight.
THE SKY AT THE SAGITTARIUS HOUR
On July and August evenings when the 18h line of right ascension is on the meridian, the sky-scene we behold is that of the Sagittarius Hour. This is the celestial hemisphere portrayed on the preceding two pages. We can walk across it in an exploration I'll call "Adventures on the Solstitial Colure."
The solstitial colure may sound like something wonderfully (or intimidatingly) profound. But it's merely the right ascension line of 18h, extended around the celestial sphere to include the 6h line on the other side. The two form a great circle through the points where the Sun stands at the times of the summer and winter solstices.
Some sights on our tour of the colure are famous. Rather low in the south the line passes between the tail of Scorpius and the spout of the Teapot of Sagittarius. It goes by the hidden center of the Milky Way as well as the Large Sagittarius Star Cloud, the Lagoon (M8) and Trifid (M20) Nebulae, and the hefty M6 and M7 open clusters. If you have an open view to the south, you'll want to linger here with binoculars and telescope a long time.
If we follow the colure northward (out of the field of the photograph above), it crosses the mouth of the Great Rift of dark interstellar clouds that splits the summer Milky Way, then takes us past large clusters and abandoned constellations. A few degrees east of Beta ([Beta]) Ophiuchi (Cebalrai or Cheleb), it cuts through a compact V of dim naked-eye stars -- the extinct constellation Taurus Poniatovii, Poniatowski's Bull. The middle star of this bull's eastern horn is a beautiful dwarf double, 70 Ophiuchi. An even lesser dwarf, Barnard's Star (see last month's Constellation Close-Up), lies near the northern star of the western horn. The latter is 66 Ophiuchi, itself a peculiar irregular variable that is also known as V2048 Ophiuchi.
Our all-sky map on the previous pages plots only the three brightest stars of the little bull. Yet it does show, just above Beta Ophiuchi, the big, loose star cluster IC 4665, visible to the naked eye and impressive at low magnifications. The cluster is featured in last month's issue, page 70.
Still higher up the colure is a surprisingly prominent little figure of dim stars in a sparse region. Our all-sky map plots its four brightest members just east of the 18h line about midway between the main patterns of Hercules and Aquila. How many of you know which "subconstellations" these stars have represented? Observe the asterism with naked eye and binoculars this month; I'll have more to say about it in the next issue.
Almost on the colure, near the dim hand of Hercules reaching toward Lyra, is the Apex of the Sun's Way. This is the point on the celestial sphere toward which our Sun and solar system are moving with respect to nearby stars.
If we want to find stars that help delineate the solstitial colure, we can use the short line forming the longest side of the head of Draco, from Gamma ([Gamma]) Draconis to Xi ([Xi]). Draco's rather conspicuous head is high in the north, nose toward Vega.
Finally, if we face north and drop our gaze from the east side of Draco's head halfway to Polaris down the solstitial colure, we'll be looking at another significant point: the north ecliptic pole, the north pole of Earth's orbit.
CLOSE-UP ON VEGA
A friend of mine calls Vega "the Sirius of Summer." The two stars are certainly leading lights of their respective seasons, but there's more. Their colors are similar, white with the barest trace of blue, and so are their spectral classes: Vega is type A0, Sirius is A1. Both are on the main sequence, so even their true luminosities and sizes are roughly similar. Vega has about 1 1/2 times the diameter of Sirius and is a little more than twice as luminous. It appears less bright only because it is almost exactly three times farther away: 26 light-years to Sirius's 9.
They are nearly opposites, however, in their positions relative to the solar system's motion. If we imagine our solar system to be a spaceship moving through the stars in our part of the galaxy, Vega would be nearly straight ahead out the front window, and Sirius would be astern. The solar system is pretty slow for a starship, however. It won't get to roughly where Vega is now for something like a half million years. And, of course, Vega itself will have moved on by then.
THE SUN, MOON, AND PLANETS IN AUGUST
The two brightest points of light in the early evening sky continue to draw closer together, as Jupiter in the southwest heads down week by week toward brilliant Venus in the west.
Venus begins August more than 40 [degrees] to the lower right of Jupiter. Look for it blazing low in the western sky as twilight descends. You can watch the apparent gap between the two planets dwindle by more than half during August.
Unfortunately, Venus is heading southward even as it brightens and becomes telescopically more interesting. So despite its increasing separation from the Sun, it is gradually losing altitude for viewers at mid-northern latitudes. Venus achieves its greatest elongation of 46 [degrees] from the Sun on August 24th, but by then it is only about 17 [degrees] above the horizon at sunset for observers at 40 [degrees] north latitude.
On the other hand, for skywatchers located on the other side of the equator around 30 [degrees] south latitude, this is a highly favorable apparition of Venus, with the brilliant planet standing exactly halfway to the zenith at sundown.
In a telescope Venus is enlarging in overall diameter while finally narrowing out of its gibbous phase. Try to determine the date when it appears to be precisely half lit.
On the evening of August 5th, use binoculars or a telescope to spot the 6.3-magnitude star SAO 119100 very close to Venus. It's only 3' or 4' to the planet's northwest (right) as seen from the East Coast; it's about 11' from Venus by the time twilight arrives in the West. Brighter Beta Virginis, magnitude 3.6, is 1 1/2 [degrees] from the planet in about the same direction.
On August 10th Venus has a nice if rather distant naked-eye meeting with the waxing crescent Moon. But its most dramatic conjunction this month is with Spica. On the evening of August 31st Venus is only 0 [degrees] .8 to Spica's south. Shining at magnitude -4.4, the planet is 140 times brighter than Spica at +1.0.
Jupiter hangs lower than we would like in the southwest at nightfall. The big planet is a noble sight in telescopes, whether or not the comet-fragment impacts on July 16th to 21st make any enduring marks on its cloud patterns. Late impacts are still possible in August; see last month's issue, page 31. Observe Jupiter as early as possible each evening.
This lordly orb picks up speed eastward in August, gliding 1/2 [degrees] north of Lambda ([Lambda]) Virginis from about August 7th through 10th and then crossing into Libra. On the evening of August 20th look for a 6.5-magnitude orange star, SAO 158554, well within the Galilean satellite system to the planet's east. Observers in Japan and Australia can watch the star's appulse with Jupiter around 9:40 UT August 21st, when the star appears only about 33" -- one Jupiter diameter! -- from the planet's northern limb.
Saturn rises in evening twilight; look for it glowing low in the east-southeast after dark. By late evening Saturn is suitably high for good telescopic observing -- a bright, sedate point of light in the southeast beside Aquarius's dim streams of starry water. It doesn't transit the meridian until well after midnight local daylight saving time. Saturn's opposition comes on the night of September 1st.
Uranus and Neptune, in eastern Sagittarius, are conveniently high in the south on August evenings. Both have interesting conjunctions: Uranus, magnitude 5.7, is 0 [degrees] .1 north of a 7.3-magnitude star for several nights around August 23rd. Neptune, magnitude 7.9, is about 0 [degrees] .2 north of a 6.1-magnitude star for several nights around the 20th. Use binoculars and the chart on page 77 of the April issue.
Pluto is hidden among the stars of Libra some 20 [degrees] to the upper left of Jupiter in early evening. Refer to the map on page 78 of the April issue if you're brave enough to seek this faint world.
Mars rises two to three hours before dawn, but it is best observed just before morning twilight. At this time we find it well up in the east among the interesting star fields of Taurus and Gemini. On August 7th and 8th it hangs between the Bull's horn tips, Beta and Zeta Tauri. Around the 20th its still-tiny disk (only 5" across) passes closely (and photogenically!) by the big open cluster M35.
Mercury is just above the east-north-eastern horizon as dawn brightens during the first few days of August. Look for the magnitude -1 world in line with Castor and Pollux on the 1st and 2nd. Mercury goes through superior conjunction on the 13th. It stays too low in the sunset glow thereafter to be seen from midnorthern latitudes even by the end of the month.
The Moon is a waning crescent near the Pleiades, Hyades, Aldebaran, and Mars on the mornings of August 1st through 3rd. New Moon occurs on the 7th, but not until the evening of August 10th is the waxing crescent rather close to Venus. The fattening Moon shines with Spica on the 11th and Jupiter on the 12th, then reaches first quarter on the 13th.
The Moon is full on the night of August 20-21, and the next evening it floats above Saturn in the southeast during evening. Then for a week the Moon sails across a dim expanse of heavens to rejoin the Pleiades on the morning of August 28th and Aldebaran at last quarter on the 29th.
TABULAR DATA OMITTED
THE MOON (Times and dates are Universal Time) New Moon July 8, 12:37 First Quarter July 16, 1:12 Full Moon July 22, 20:16 Last Quarter July 30, 12:40 New Moon Aug. 7, 8:45 First Quarter Aug. 14, 5:57 Full Moon Aug. 21, 6:47 Last Quarter Aug. 29, 6:41 Greatest and Least Distances Apsis Distance Diameter Apogee July 3, 5h 404,677 km 29'32" Perigee July 18, 18h 367,865 km 32'29" Apogee July 30, 23h 404,086 km 29'34" Perigee Aug. 12, 23h 369,464 km 32'21" Apogee Aug. 27, 18h 404,343 km 29'33"
Light-Pollution Notes: International Progress
THE UNITED STATES is not the only country where activism against light pollution is astir. In the United Kingdom, the Campaign for Dark Skies (CfDS) is active as a joint project of the British Astronomical Association and the International Dark-Sky Association (IDA). I'm impressed with its accomplishments and its literature; dark-sky advocates everywhere can benefit from studying its methods. The coordinator of CfDS is Bob Mizon, 38 The Vineries, Colehill, Wimborne, Dorset BH21 2PX, United Kingdom.
Efforts to reduce light pollution are also being mounted in Quebec, Japan, Australia, Belgium, Denmark, Italy, Spain, Hungary, Brazil, and surely other countries. We are still in the early stages of the battle, but the movement for good lighting is growing rapidly.