The Time of Day, The Day of Time
By Julian Kane
The 24-hour so-called diurnal day is based upon the rotation of the Earth about its axis and is determined by the elapsed time from one solar noon to the next. Solar noon occurs when the Sun appears highest in the sky and it coincides with clock-noon only along the central meridian of any 15°-degree wide time zone. There are 365 1/4 diurnal days in a year.
The Earth rotates almost 361° (not 360) during a diurnal 24-hour day. Our planet must spin 1° more than 360° (in order to compensate for its having moved 1° in its orbit during that 24 hours) before another alignment with the Sun can occur to complete the diurnal rotation cycle.
The 360° sidereal day takes 23 hours, 56 minutes and 10 seconds to transpire. It is used mainly by astronomers to keep track of when celestial objects are expected to reappear "daily" at the same position in the sky after the Earth has rotated precisely 360° about its axis.
For those readers who find 24 hours insufficient to complete their day's work, I have good news-the diurnal day is getting longer! In fact, scientists have known since 1963 that the day has been gradually lengthening over the past 600 million years, and that this is expected to continue in the future.
Corals and other shelled invertebrates incrementally add material to their external skeletons constantly throughout their lives, but at periodically varying rates. This shows up on their shells as daily and yearly growth line markings. The diurnal additions reflect different daytime and nighttime feeding and shell-growth activity. The discernible annual growth line markings reflect yearly changes in metabolic production of shell material that is related to seasonal reproductive cycles and seasonal availability of food.
While an average of 365 small daily growth lines can be counted on presently living shells between the larger yearly increments, fossil shells and corals contain a greater number of daily lines within the ancient annual shell markings; indicating a slowing of the Earth's rotational velocity due to tidal friction, and an average decrease in the number of days per year of 20 days every 200 million years. According to diurnal and annual growth lines on fossils, the year had 425 days 600 million years ago, 405 days 400 million years ago, and 385 days 200 million years ago.
Astronomers had for many years been puzzled as to why eclipses (which are supposed to reoccur on a fixed schedule) were always slightly late. Mathematicians tackled the problem and determined that the discrepancy could be accounted for if the Earth's rotation rate was slowing enough to be losing 20 days per year every 200 million years. Geophysicists then figured out that the frictional braking effect of tides and surf waters moving back and forth across shorelines around the world would produce just such a rotational deceleration and loss of 20 days per year every 200 million years.
Finally, geobiologists (paleontologists) compared the annual number of daily growth lines on modern shells with fossil ones that lived at different times in the past and provided the proof that solved the enigma. A 425-day year denoted a 21-hour day 600 million years ago, a 405-day year indicated a 22-hour day approximately 400 million years ago and a 385-day year derives almost to a 23-hour day 200 million years ago. Ah, the fascinating stories fossils can tell!
A new theory was recently proposed in Scientific American that the Moon originated 4,500 million years ago from a shattering collision between the Earth and a huge massive object. The proponents calculated that the cataclysmic impact left our planet spinning on its axis once every two hours, but they are puzzled as to how the Earth's day could have lengthened from two hours to 24 hours. As you might surmise, I have written to Scientific American to tell them.
This weeks is Regents Exam Week for many high school students across New York State. Although I do believe that the number of Regents exam takers this week is quite small compared to the large numbers sitting for the various Regents in June, still, a fair number of students are facing some of those exams this week.
And the Regents exams are far more important than many of us realize. I, for one, had some ''eye-opening'' experiences during the past year, and I thought I would pass along some of the information.
My family just went through the college admissions process with our younger daughter. And while four years ago our older daughter would consider only one school (and, fortunately, she was admitted quite early), the younger one had no preconceived opinions. So last spring we began the college search, visiting several campuses.
Most of the schools we visited were not located in New York State, but most admissions officers seemed to know all about Regents courses and Regents exams. At the very least, the schools all seemed to acknowledge that a Regents class meant a ''college prep'' class, a course weighted more heavily than a ''general course.''
When we visited a university located in New York State (though not a ''state school''), that school's officials spoke about the importance of the ''Regents track'' and passing Regents exams in order to receive credit for having taken the courses.
We were a little surprised, especially since so many people are quick to say ''A Regents diploma isn't important.'' Maybe a Regents diploma is not crucial, but more schools than we ever imagined place a fair amount of importance on Regents exams and Regents courses.
Yes, we were surprised, but I bet our Superintendent of Schools, Dr. William Shine, is not at all surprised. After all, he is the one who has consistently stressed the importance of an outside evaluation such as the Regents examinations. Dr. Shine is the one who has often said that those exams should ''count'' in the final course grade even more than the traditional one-ninth used in our school district. And Dr. Shine is the one who has always spoken of the value of Regents classes and the resulting Regents diploma.
We shouldn't have been so surprised to discover that Dr. Shine was so right!
Good luck to all of the students who face the challenge of a Regents exam this week.
---Wendy K. Kreitzman