Winter solstice

I certainly hope that the weather for:
observing is better this month than it .
was during. November, While I enjoy

_ reading about astronomy, J find that a
few clear nights really help me to keep
track of the changes that occur in the
sky from week to week, and there really
weren’t many opportunities for this last

. month. On-the few occasions when it
was possible to see stars, they were a

by Fars . very welcome sight.. .
The winter solstice occurs at 7:28 a.m. |
on December 21 this year. This ‘is the time when the Sun appears _
as far south as possible, and for us in Terrace the day is.the ‘ a
shortest of the year. Because the Sun is furthest to the south, this. iim
day is also the southern hemisphere summer solstice and the
longest day of their year. Regardless of which hemisphere is be-
ing considered, the length of day changes most rapidly around
the time of the equinoxes (about September 21 and March 21)

and is slowest at the solstices (about June 21 and December 21).

The times of these equinoxes and solstices set our calendar —
if they shifted gradually from year to year then eventually the
winter solstice would no longer occur in December, Instead it
would gradually move into January, and then after about 120
years February, and so on. The reason for this shift is that the
year isn’t an even number of days but instead 36514, and this ex-
tra one quarter day means that if a year begins at midnight ona .
particular day, the next year will begin on the same date but one
quarter day later. After four years the year will begin a day later
than it did on the first year. This is the reason for leap years. In
these years the extra day is inserted so that the beginning of the
year shifts back to its original starting date.- .

The fact that the calendar had to be reset has been known for
centuries. Our present Gregorian calendar, for example, has been

‘in operation for about 400 years, and the Julian calendar that

preceeded it (and is the same in principle, just a little less ac-
curate) was introduced at the time of Julius Caesar. Even before
that, ancient astronomers. knew. that the year.was.not.an-even ——

‘umber of days, and it is thought that some of the ancient

astronomical monuments were used for the purpose of resetting ° NP-4540 RDF
ancient calendars. Stonehenge, in England, is probably the most _ -
famous of these monuments and the possibility of its use for this G. ahead and taka abite, °

purpose has been well documented.
_ If the weather co-operates you can éheck the date of the winter
solstice yourself. To do this you need nothing more complicated
than a stick, although a sheet of paper may be helpful too, If
you place the stick upright in the ground at a location where it ©
won't be disturbed, you can mark the length of the shadow at
different times of the day over a period of several days. The  § ff
shadow will be shortest at noon on any day, but the length of the
shortest shadow will vary from day to day. On the date of the
winter solstice the noon shadow will be the longest of any
(because the Sun is furthest south), though it will still be the
shortest shadow of that day. If you have a paper around your
stick then you can mark the positions of the shadow on the days
near the winter solstice. In this way you can verify the December
2lst date of the winter solstice.
There is one difficulty in using this method for finding the date
of the solstice (other than the need for good weather, that is).
This difficulty is that the change in south position of the Sun at. -
noon is smallest at the solstices — this is the same reason that the
change in the length of the day is so slight at these times. For this
reason the length of the shadow at noon doesn’t change very
much at all for the few days around the solstice, so if you try this
experiment you must measure the shadow length carefully. .
A sundial shadow can be used in the same way as that of a stick. _

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This month there are several planets that are well positioned
for observation. Though it-is not nearly as bright as it was in -
September and October, Mars will still be easy to observe because
it is high in the southeast sky in the early evening and it sets after
midnight. .

Jupiter will rise in the east at about sunset. Jupiter is Fun to
look at because there is always something interesting happening
on or around this planet, With a small telescope you can see light.

‘and dark bands on its surface, and even with binoculars. you can
see the four largest of Jupiter’s moons. If you watch them overa  ~
period of one or two hours, you will even see changes in their
positions. Jupiter is very well positioned for observations during
December.

Venus will be in the southeast just before sunrise during the
first part of December, It is moving closer to the Sun however,
and it won’t be visible in the latter part of the month, -

The Terrace Astronomical Society will be meeting on Tuesday,
December 13 at 7:30 p.m. in the Terrace Public Library. If you

~ are interested in learning ‘more about astronomty, or if you

already know a lot and would like to share your knowledge with

'. others, you are most welcome. If the weather is clear, telescopes

will be set. up near the library after our meeting. As well, the

_ Astronomical Society provides the opportunity for you to look

through telescopes every clear Tuesday night from 8 to 9:30 p.m.

The. telescopes are set up near the public library at these times.

Wi LKINSON

~ 4552 LAKELSE AVENUE
TERRACE, B.C. V8G 1P8

604/638-8585 —

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