Sky & Telescope June 1985 [antikvár]

More Sites for Observing Halley's CometEdward M. Brooks, Boston CollegeAlready, many observers are plan-. ning trips to southern locations to view Halley's comet. In the October, 1984, issue, page 304, I described some of the world's best observing sites for the comet's upcoming apparition. Here I'll consider only places north of the equator, for the benefit of observers with limited time or resources. My selection of sites in the United States and Mexico is based on climatological data. (Northern Hemisphere sites in Africa and Asia, besides being more difficult for many to reach, are astronomically not as good as the South American sites I listed previously.)The best times to see Halley will be when the Moon is either new, a thin crescent, or below the horizon. If you want to view the comet before dawn, the Moon should be in the evening sky; for viewing after dusk, the Moon should be a morning object. A summary of the Moon's phases and other conditions affecting the comet's visibility is given in the table above. (For more details, see "Brighter Prospects for Halley's Comet" on page 9 of the January, 1984, issue.)My Northern Hemisphere observing sites were chosen for best conditions in early April, 1986, when Halley is predicted to put on its best show. Unfortunately, this is when the comet reaches its southernmost declination, 48° below the celestial equator; it will probably not be visible except for the tail, which will extend higher than the head, to anyone living north of 40° latitude.City lights and haze can pose serious interference when the comet is at low elevation angles. Likewise, poor seeing can be expected in strong gusty winds or cold unstable air. Therefore, sites with warm, steady air that are far enough south to place the comet above haze layers are best. For example, at any given date in the spring of 1986, for each degree of latitude we move south, Halley will appear 1° higher above the horizon.For the table of recommended sites on page 486, only locations south of 35° north latitude were considered. In addition, only sites with a mean cloud cover of 4 tenths or less (as recorded at local airports) for April were considered. Major cities are not Usted because of light pollution. Although airports are also brightly lighted, their surrounding suburbs are darker and would have nearly the same cloudiness.The table includes seven places in theNORTHERN HEMISPHERE VISIBILITY OF COMET HALLEYComet's timeBest weelcsNew Moon 19S6 Full Moonof visibilityfor observationJan. 10Jan. 26After duskFirst 2 weeks of Jan.Feb. 9Feb. 24Lost in Sun's glareMar. 10Mar. 26Before dawnMiddle 2 weeks of Mar.Apr. 9Apr. 24Before dawnMiddle 2 weeks of Apr.May 8May 23After duskFirst 2 weeks of May.United States and 12 in Mexico. There are undoubtedly many others that would be just as good, if not better, for observing the comet. Their omission from the table is merely due to the lack of readily available climatological data.Because of Halley's low elevation, topography of the observing site is another important consideration. Ideally, the southern horizon should be flat and as low as possible. If there are mountains to the south, their apparent heights should be compared beforehand to the comet's predicted elevation. In addition, clouds often form over mountains when air moves up their windward slopes to higher ahitudes, where it cools to its condensation point.Another hazard is early morning fog that forms in moist air masses after the ground beneath cools overnight. To avoid this fog, choose a dry region such as a desert. In my earlier article I noted that such dry areas may be found on the lee sides of mountains or in the subtropics at latitudes 20 to 30° south, where persistent areas of high pressure exist.For practicality, an observer will want to have frequent air service to the airport near a chosen site. Good accommodations are also desirable, since an observer may have to wait a few days for a succession of clear nights.EASTERN UNITED STATESThe April mean daytime cloudiness, measured by the percentage of sky, affected, for the southeastern United States is 0.4, with Florida being the best accessible region. For those traveling to the state's northern regions, Jacksonville is the closest city, but the comet will be higher in the sky as seen from southern Florida. Best of all are the Keys. Not only ate they the farthest south, but they have the best predawn sky. As small islands surrounded by warm ocean water, the Keys are free of cool ground fogs typical of moist continental areas before sunrise. But because the Keys are limited in ac-commodations, it might be advisable to book a hotel room on the mainland southwest of Miami. Then, if the comet does not show up well on the first clear night, you could drive to the Keys on the next clear night.Even at worst, April's weather in the Keys will not include hurricanes. Floods are possible but unlikely. In April, 1941, a rare downpour dumped over a foot of rain on Key West. But even a repeat of this calamity in April, 1986, would not last more than a few days, whereas the comet will be on display for much longer.For January observations, Florida also offers the best eastern U. S. sites. The Keys are most recommended solely because of their comfortable nighttime conditions. Key West is the only U. S. city (excluding those in Hawaii) that has never recorded freezing conditions, and temperatures are not likely to fall below 50° Fahrenheit.WESTERN UNITED STATES