Sunday 19 December 2010

Astronomy 2 for 1: Winter Solstice and Lunar Eclipse

It's all coming together on Tuesday, December 21, 2010. We get not one but two events for those fond of astronomy. For the uninitiated, let's look at a couple of explanations.

Winter Solstice
This is the point in time when the Earth is the farthest away at its maximum tilt of 23 degrees, 26 minutes. For all of us, the days have been getting shorter and nights have been getting longer up to this point, the winter solstice. Afterwards, the days will begin to lengthen again and the nights will shorten. The exact date of the winter solstice may be December 21 or 22 depending on the shift in the calendar.

If you haven't guessed, there is a summer solstice which is June 20 or 21. This is the longest day of the year for us. And don't forget that down under, for those in the southern hemisphere everything is the opposite. Our longest day is their longest night. And by the way, even though the length of the days and the nights is the opposite in the southern hemisphere, the water in a toilet does not drain in the opposite direction. That's a myth about the Coriolis Effect operating on draining water.

Just to round things out, the middle point between the two solstices is called the equinox and we have two of them. There is the March equinox or the vernal equinox and then the September or autumnal equinox. According to the calendar, they may be March 20 or 21 and September 22 or 23.

Now how does all this work in respect to the sun? For an equinox, it's simple. That is the point when the sun is directly over the equator. For our summer solstice, this is when the sun is the farthest north and directly over the Tropic of Cancer which is at latitude 23 degrees north 44 minutes. For our winter solstice, the sun is over the Tropic of Capricorn in the southern hemisphere at latitude 23 degrees south 44 minutes.

True Story
Years ago I doing a long car trip in June with an acquaintance, Mike. We were chit-chatting back and forth trying to wile away the time. At some point, realising what the date was, I wished Mike a "Happy Summer Solstice". He didn't know what the word solstice meant so I explained it to him. For the next three days he kept chuckling away about my Happy Summer Solstice greeting. He thought this was quite amusing; ah, fun with words.

The next time you're in a card store and want to do something original and unusual, pick yourself up a blank card and fill in a Happy Solstice or Equinox. Let's see if the recipient even knows what that is! :-)

Hey buddy, got a minute?
I used the expression 23 degrees, 26 minutes to describe the tilt of the Earth on its axis. What the heck is a minute?

We all know the word minute relates to time but as it turns out, it is a term also used in describing angles. If we look at the equator running around the middle of the globe, we have 24 hours with each hour divided into 60 minutes. In that case we use longitude to describe the East West displacement and use minutes and seconds to further define that displacement. My home town of Toronto is located at 79 degrees, 20 minutes and 26.47 seconds west longitude and 43 degrees, 42 minutes and 59.72 seconds north latitude. By the way, at the equator one minute is equal to about one nautical mile which is 1,852 metres or approximately 6,076 feet.

Don't forget that latitude is the angular distance north and south from the equator and longitude is the east and west. Ah yes, zero latitude is the equator but zero longitude by agreement is the line which goes through Greenwich, England, the Prime (Greenwich) Meridian; Meridian is a synonym for "line of longitude". Why Greenwich? We had to start somewhere!

Lunar Eclipse
Gee did you think you'd be covering all this astronomy stuff today? :-) So, December 21 is going to be our shortest day of the entire year. However added to this little treat is a lunar eclipse. According to NASA, we in North America seem to be well placed to observe this phenomenon; a ringside seat.

A lunar eclipse is when the moon passes behind the Earth and the Earth blocks the rays of the sun. For those in the known, you can get into the three parts of a shadow with the umbra and penumbra. What? Isn't a shadow a shadow? Well, not quite because we make the distinction between a point source and a non-point source of light. [groans] Oh brother, is this getting complicated? Yes and no. Besides, thank your lucky stars we're leaving out the antumbra!

Normally we think of a light source as something like a light bulb. It's small; it's a point source; just a single point of light. However the sun is massive; it is a non-point source of light. That means that in order to block out the sun, you need something big enough to completely block all of the sun and considering just how big it is, that's a tall order to fill.

In a nutshell, the umbra is the true shadow where the penumbra is a partial shadow. NASA has a good diagram of what's happening which will explain everything to the reader. I quote NASA's schedule of the event:

Penumbral Eclipse Begins: 05:29:17 UT
Partial Eclipse Begins: 06:32:37 UT
Total Eclipse Begins: 07:40:47 UT
Greatest Eclipse: 08:16:57 UT
Total Eclipse Ends: 08:53:08 UT
Partial Eclipse Ends: 10:01:20 UT
Penumbral Eclipse Ends: 11:04:31 UT

UT? What the heck is that?

UT stands for Universal Time and is the time at the Prime Meridian at Greenwich. There are other details which confuse the H out of me so in keeping with the KISS principal; I'm restricting myself to that. Eastern Standard Time is UT minus 5 so in the above list of times from NASA, the Total Eclipse begins at 7:40:47 UT and minus five brings us to 2:40:47 am EST. Okay why did those smarty pants scientists choose to give the schedule using UT? Because it's universal! All you need to know is your own offset to UT and you're good to go. EST = UT-5 or UT minus five.

On Tuesday, the show starts at 1:33am. The full eclipse lasts from 2:41am until 3:53am and the show is over at 5:01am. That's 72 minutes of the moon being Earth's umbra.

So don't forget to take advantage of this pairing of phenomenons. Phenomena? Apparently the last time a lunar eclipse occurred precisely on the winter solstice was 1554 so that means the next time is 2466. Let's not miss this one as I'm pretty sure none of us will be around in 2466. I guess that means it's worth getting up at 2am! :-)


Wikipedia: Winter Solstice

Wikipedia: Lunar Eclipse

NASA:Total Lunar Eclipse


Site Map: William Quincy Belle

No comments: