djb@cbosgd.UUCP (David J. Bryant) (01/16/85)
I don't think many of us can get to Venus to watch Halley's, but things on Earth won't be all that bad, provided you have enough money to afford travel to the southern hemisphere. True, Halley's won't be as spectacular in 1985-86 as it was in 1910 (or before), but it won't be the true dud that folks seem to think: > Halley's Comet will be at its most spectacular at it's (sic) closest > approach to the Sun. It will be on the opposite side of the Sun from > Earth at that time. Well, this is pretty much always true. However, it's also always true that no matter where the Earth is, you can't see anything close to the Sun. (We'll exclude planetary transits from this discussion.) The Sun is prety bright, and generally can be counted on to ruin the view of anything that is anywhere close by. This is by no means a unique aspect of this particular reappearance of Halley's Comet, and should be disregarded. What is important is how close Halley's will pass to Earth, not the Sun. In a fit of cleverness, astronomers have defined three classes of return appearances of Halley's Comet: 1) The comet passes very close to Earth on it's inbound trip, but pretty far from Earth on the outbound leg. 2) The comet passes very close to Earth on it's outbound trip but pretty far from earth on the inbound leg. 3) The comet is about equally far away from Earth on both the inbound and outbound part of it's orbit. The respective geometry of Earth's orbit and Halley's orbit make these cases the only possiblities (you can't have a case where the comet is close on both the inbound and outbound legs). Obviously you get much better views if the appearance is either a Type 1 or Type 2 than you do if it's a Type 3. The 1910 appearance was a Type 2, by the way. What we have in 1985-86 is a Type 3 appearance. The result for us earthlings is that we get a pretty good view of the comet on the way in (but not as good as a Type 1), and a pretty good view of the comet on the way out (but not as good as a Type 2). Happily, the combined view is better for Type 3 than the other two, since in Type 1 you have a bad view of the outbound trip and in Type 2 you have a bad view of the inbound trip. All this applies regardless of where you on Earth you are observing from. Beyond basic issues associated with where Halley's orbit intersects Earth's orbit, there are viewing parameters related to the respective tilts of the two orbits. For most of it's "year", Halley's comet is below the plane the Earth's orbit (the "Plane of the Ecliptic"). As it approaches the Sun, however, it crosses the Earth's orbit, travels above the plane of the ecliptic, passes close around the Sun, then heads back out, finally crossing the Earth's orbit and falling below the plane of the ecliptic. Whether or not we get a good view in the northern hemisphere is strictly dependent on where the Earth happens to be when Halley's comet crosses our orbit and is near us and the Sun. The same is true for the southern hemisphere, although to a different degree. If you do some calculation you discover what the conditions are for northern hemisphere observers for the 1985-86 return. Before perihelion, northern hemispher observers have the best view of Halley's. That's right, NORTHERN HEMISPHERE observers have the better view! Halley's comet will appear higher in the sky, for more of the night, and be viewable for more days by northern hemisphere observers than southern hemisphere observers before perihelion (Feb 9th, 1986). We get the best seats for watching Halley's approach Earth and grow brighter and more noticable. That's the good news, and it's not particularly widely known. As a bonus, for the most part, Halley's is an evening object before perihelion. No getting up at the crack of dawn to watch the comet. You can go out just before sunset, set up your camera/telescope/binoculars and enjoy a summer or fall evening watching for Halley's Comet. Now for the BAD NEWS. After perihelion the shoe is on the other foot, and southern hemisphere observers have the better view. At best, folks in the northern half will only catch fleeting glimpses of Halley's comet low on the southeastern horizon just before sunset. Given the conditions of most people's low horizons - trees, lights, etc. - the view will be pretty bad. This is the bad news that northern hemisphere observers has been stewing over of late. To make matters worse, Halley's will be slightly brighter after perihelion, and will not really "grow" a spectacular tail until after it has passed around behind the sun. Southern hemisphere observers will be able to watch the comet at its best, while folks up north suffer. In general, Halley's will be a morning object during this period, although there is a stretch of several days when southern hemisphere observers will be able to all night long! Overall, it's true, southern hemisphere observers will have a significantly better view this time, but it's not totally one sided. The solution is to make plans to do your observing in the northern hemisphere this summer, fall and early winter. Around the end of January 1986, pack your bags and make plans to travel to Australia, Africa or South America. Spend the month of April, 1986 somewhere around 30 degrees south latitude. For a bonus, stop in Tahiti in late April for a total lunar eclipse that will take place when Halley's Comet is only 15-20 degrees from the Moon. Sorry for the length of this. I could actually go on for much longer but will quit. I have further details on all the general statements I have made above, plus can point you to several resources that might make your observing easier and more enjoyable. The bottom line is not to write this appearance of Halley's Comet off altogether. Despite the unfortunate alignment of Earth, Sun and Halley's, it still has the potential to be a once-in-a lifetime event. There will be wonderful views of Halley's from all over (provided you don't have continuous cloud cover). Start planning and practicing your comet observing/photographing skills now and be ready come summer when Halley's will be bright enough to see in large amateur telescopes. David Bryant AT&T Bell Laboratories Columbus, OH (614) 860-4516 (cbosgd!djb) ps: For the 2061 return, we can probably expect folks to travel up to space stations or to the Moon for better views than we could ever get here on Earth. This'll be the last time we'll be "grounded".