Celestial topics include snowflakes, comets, leap years and a preview of the astronomical events for 1996.
The Star Gazer's Almanac of Astronomical Events
You've had your fill of it by this time of year, I imagine; it's heavy to shovel from your driveway in the early spring; it's wet and cold on your head, and dangerous on the roads. That's all that many adults think there really is to say about snow.
But don't all of us retain the child inside who found snow not just a way out of school but a marvel, an enchantment, in fact a world-creator?
It really is as if we live in a different world when snow surrounds us. The ugliest landscape can be smoothed and whitened and set to glitter. Trees draped and clad in cloaks of heavy snow take on a new majesty, and shut off the outside world to form cathedral aisles for us to walk down. Snow alters not just the visual but the auditory realm in which we live.
Snow on the ground, on the trees, even snow in the air, absorbs noise. And I think we should usually be glad to have ourselves insulated from most of the cacophony of today's society.
Another amazing property of snow is how much of it is nothing but air. The air-to-water ratio of snowflakes is such that on the average, 10 inches of snow can be melted down to one inch of rain. But some snow has been so fantastically airy that what would be one inch of rain is, on the ground, several hundred inches of snow!
The gentle fall of snowflakes is so lovely and entrancing that even the sourest cynic must have trouble ignoring or scorning them. Get a magnifying glass and look at them as they settle on your coat. Sometimes you will find that they are not the traditional flake form, the "snow-star." They may be clumps of crystals: snow "aggregates." They may have irregular forms, as if they were shattered relics. Or, if there is warm air not far aloft (the snow may soon turn to rain), they maybe needle-shaped. Those needles may even conduct electricity in thunder-snowstorms, forming lightning.
Some of the other forms of snowflake or snow crystal are quite beautiful, but of course everyone's favorite is the six-pointed snow-star. Listen to Henry David Thoreau's words about them:
"Nature is full of genius, full of the divinity; so that not a snowflake escapes its fashioning hand … A divinity must have stirred within them before the crystals did thus shoot and set. Wheels of the storm-chariots. The same law that shapes the earth-star shapes the snow-star. As surely as the petals of a flower are fixed, each of these snow-stars comes whirling to earth, pronouncing thus, with emphasis, the number six."
Are You Ready for Bissextile Day?
Speaking of numbers, leap day comes but once every four years ... right? Not always. The rules governing leap days are actually more complex than most people realize, as I'll explain in a moment.
They are complex, but at least they aren't as complex as the patterns of leap weeks and leap months that have been required in the calendars of various cultures throughout history. And in modern times we have added a confusing innovation: "leap seconds," which get added to the end of the first half or second half of a year every few years, to keep official time within X-second of the gradually and irregularly slowing rotation of our planet Earth. (By the way, the organization that decides when these leap seconds are needed is known as the International Rotation Service ... really!)
Having a leap day in it makes a year become a leap year, 366 days long instead of 365. Leap years are needed to keep our calendar in synch with the seasons because the number of days (24-hour periods or Earth-rotations) in a year (one complete circling of the Earth around the Sun) is not an even number. It is about 365-1/2 days. Thus, adding one day every four years should set things (almost) straight.
It was Julius Caesar who decreed that the Roman calendar would start using one leap day every four years. By the time he did, that calendar was already so far behind the seasons that 67 extra days had to be inserted after November 46 B.C. (Incidentally, Caesar was killed before he could see the first use of his leap day.). The Roman system for adding the leap day was strange indeed to our way of thinking. They did not add a February 29. Instead, they repeated the day February 24 twice in a leap year. Since the Romans called February 24 the sixth day before March I (counting March I as one of the days), the leap day was dies bissextus, twice sixth day. And so even today the term bissextile is sometimes used for leap.
But a few paragraphs back I said that leap day doesn't always come once every four years. The institution of the Gregorian calendar by Pope Gregory XIII in 1582 included a new leap year rule. Century years like 1700, 1800, or 1900 would not be leap years, even though they were in line in the four-year cycle ... except for century years evenly divisible by 400, which would be leap years. Got that straight? There was no leap year from 1896 until 1904 because 1900 was a century year not evenly divisible by 400. But 1600 was a leap year. And 2000 will be a leap year too.
Have we gotten to the end of the complications about leap years? Not yet. It has been decided that to bring the calendar year even closer in step with the true year another rule will be used: the years 4000, 8000, etc., though evenly divisible by 400, will not be leap years. Abiding by this law will make the calendar accurate to within one day in 20,000 years. And there will be no date called February 29, 4000unless perhaps somebody changes the rules again in the next 2,004 years.
Month of Comets
Our Almanac page lists several fine astronomical events for February and March. And the accompanying sidebar gives an overview of such happenings to look for during all of 1996. But March 1996 is several other things to amateur astronomers. It is a time of anniversaries for two of the most interesting comets of our past. And it is a time to begin the countdown for a new comet, which in March 1997 may blaze more impressively than any other in our century.
Unlike meteors (so called shooting stars) which are tiny bits of space rock burning up in Earth's atmosphere in a few moments, comets are mountains of ice millions of miles off in space that remain visible for weeks or months at a time. When the icy comet "nucleus" comes far enough into our inner solar system and gets stirred by heat and other solar radiation-presto! A cloud of dust and gas hundreds of thousands of miles wide may form the "head" of the comet. The pressure of sunlight and the flow of atomic particles from the Sun (the "solar wind") may push dust and gas back from the head to form a streamer or fan millions of miles long-the "tail" of the comet.
Many of you readers probably remember the great public interest at the previous return of Halley's Comet, in 1985-86. Unfortunately, you may also remember not seeing the comet itself due partly to this being a relatively dim return (the comet didn't get close enough to Earth)and even more to the fact that city "light pollution" ruined so many people's view. But Halley is such a historic object that its visit was still a memorable event. Halley's brighter return comes in 2061.
If you want to hear about a comet that looked truly breathtaking, you have to go back 20 years, to March 1976. The head of Comet West was a mass of gold in which a brilliant point of light shined and from which a pair of awesome dissimilar tails extended up out of the morning twilight. Comet West was easily visible to the naked eye for weeks but not many folks are up at 4 to 5 a.m. and I think astronomers didn't want to publicize it too much in advance for fear it would disappoint, like the most famous of fiasco comets, Kohoutek, had two years earlier. But the glorious Comet West was no disappointment.
Now, as February 1996 ends, far out in space a new comet is swooping up in front of Jupiter. It was discovered by amateur astronomers Alan Hale and Thomas Bopp last July, and if astronomers' predictions are right, Comet Hale-Bopp may outshine the brightest stars as it enters the evening sky in March 1996 or 1997. I will be keeping you posted here, as this potentially stunning object heads inward towards the general vicinities of the Sun and Earth.
Astronomy Preview of 1996
This year's most exciting events for watchers of the night sky include two total lunar eclipses, two strong meteor showers in a moon-free sky, Venus' greatest showing as Evening Star, and the approach of Comet Hale-Bopp.
Eclipses. The year's first total eclipse of the Moon is seen in its entirety only from the easternmost U.S., but most Americans get at least some kind of look at it after sunset on April 3. The second total lunar eclipse occurs with the Harvest Moon itself, on September 26, and will be well visible across more of the country. This second eclipse will feature a special bonus: the darkened and probably cheerfully reddened Moon not far from Saturn on the day of the latter 's "opposition," its best positioning of the year. The partial solar eclipse of October 12 will be visible only from northern Maine and southeastern Canada and there will be only a tiny "bite" taken out of the Sun as it comes up.
Planets. Saturn is at its best in September, its rings tilting back open somewhat for telescopic viewers in the months after they are edge-on to Earth one last time (on February 111. Jupiter, now being orbited and studied by the Galileo spacecraft, is at its brightest and visible all night long on the Fourth of July. Mars doesn't get conspicuous until near year's end, but Venus is magnificent all spring and most of the summer. Venus is visible for up to four hours after sunset in early spring, then zooms past the Sun to rise almost as long before dawn in the summer. Mercury is best glimpsed low in the west about 45 minutes after sunset around April 23, and low in the east about 45 minutes before sunrise around October 3.
Meteor Showers. Meteors are streaks of light caused by bits of space rock burning up in Earth's upper atmosphere. On certain nights each year, denser streams of these little particles encounter Earth and the result is increased numbers of meteors-also called shooting stars and falling stars-radiating from some point among the constellations. Such a display is called a meteor shower, The best views of meteor showers come when there is not bright moonlight in the sky during the peak hours of the shower. That condition is met this year for the two showers that are usually the year's strongest: the Perseids and Geminids. The Perseids should reach their peak in the predawn hours of August 11 and 12, and the Geminids in the middle of the night of December 12-13.
Approach of the Comet. The monster comet Hale-Bopp is still more than 500 million miles from Earth and the Sun in the opening weeks of 1996. In summer, though still far beyond the main asteroid belt, it may become just visible to the naked eye, perhaps farther out than almost any other comet in history. By November or December Comet Hale-Bopp could already be a rather impressive object low in the west sky after sundown—with its nights of true glory still to come.