JOURNEYS

(originally run: May 20, 2005)

There is nothing more sad or glorious than generations changing hands.

≠ John Mellencamp

This author and his family are experiencing one of the more bittersweet experiences of life this week. His older son is graduating from high school, effectively bringing to an end the childhood phase of his life and opening the door to young adulthood and all of its challenges and promises.

The events this week are bringing back recollections of this author's own high school graduation 29 years ago, and all the experiences of life that have transpired since then. These have included triumphs, tragedies, mistakes ≠ and, hopefully, learning from those ≠ and all the various other events and occasions that make up this thing we call ≥life.≤ While this author would like to believe that there are still several decades left for him to enjoy life and many things left to accomplish while doing so, there is nevertheless a sense of ≥passing the baton≤ as he watches his son take his walk across the stage.

The passage of time is linear, but in many senses it can be considered cyclical as well, for example, the events this week being reminiscent of this author's similar experiences a generation ago, and so on in turn with the generations that preceded his. Likewise, many of the events taking place in our skies are also very cyclical in nature. We only need look at our monthly cycle of moonphases as the moon orbits around the Earth, and our annual cycle of seasons as the Earth makes its orbit around the sun, to see this taking place.

There are many kinds of astronomical phenomena that recur at regular (or at least semi-regular) intervals, and this author has been studying the sky long enough that he has begun to witness several of these repeating events. For example, solar and lunar eclipses repeat in an interval known as a ≥saros,≤ each particular leg in such a cycle taking slightly over 18 years to complete. (There are several such saros cycles going on at any one time, which explains why we have eclipses of some kind taking place every year.) Next March the path of a total solar eclipse will cross parts of Africa and Asia (and advertisements for eclipse tours are already appearing in astronomical magazines); this event is part of the same saros series that produced an eclipse on this author's 12^th birthday, and which he remembers viewing as a partial eclipse from here in southern New Mexico.

The planets' motions in their respective orbits also lead to repetitions of their viewing circumstances. The planet Saturn, for example, is now visible in our western skies after dusk, forming a distinctive pattern with the bright stars Castor and Pollux in the constellation Gemini. Saturn's orbital period around the sun is 29 years, and thus it is presently occupying almost the exact same spot in the sky that it was occupying at the time of this author's high school graduation in 1976. Due in large part to the flybys of Saturn by the twin Voyager spacecraft in the early 1980s and to the Cassini spacecraft which is currently in orbit around Saturn, our present knowledge of the ringed planet and its environment is, of course, vastly greater than it was at that time.

The periodic comets that return every few years also illustrate the cyclical nature of astronomical events. One such comet, Giacobini-Zinner, which has an orbital period of 6.6 years, was a fairly bright object during its return in 1985, right at the time of this author's marriage. It has since returned in early 1992 ≠ at the same time as the birth of this author's second son (although the comet was badly placed for viewing that return) ≠ then in late 1998, and is now once again visible in moderate-sized telescopes as it travels through the constellation of Pegasus in the morning sky. Another comet that should become visible within the next month or two is Hartley-IRAS, which has an orbital period of 21 1/2 years; its last return, at the very beginning of 1984, took place at the same time this author began working at the Jet Propulsion Laboratory in California.

Despite the cyclical recurrence of these and other phenomena, time nevertheless continues its linear march forward. As illustrated by the above example of Saturn, we should expect that our knowledge about the celestial objects around us, along with the many other phenomena we see here on Earth, will continue to increase. We can also work to ensure that, as the members of this author's sons' generation begin to take their place in our world, they have the capability, and the opportunity, to meet and overcome the many technological and sociological challenges our society faces. May their generation achieve the peaceful and just society that has so far eluded us, and let us do everything we can to assist them.

As we travel to whatever future awaits us, the cycles of the heavens will continue their course. Perhaps a future return of Comet Giacobini-Zinner will herald the marriage of one of this author's sons. And perhaps the next passage of Saturn by Castor and Pollux 29 years hence will herald the high school graduation of one of this author's grandchildren, and the passing of the baton of life to yet another generation.

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