The Longest Day

The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. This happens twice each year, at which times the Sun reaches its highest position in the sky as seen from the North or South Pole respectively.

The summer solstice is the solstice that occurs in a hemisphere's summer. In the Northern Hemisphere this is the Northern solstice, in the Southern Hemisphere this is the Southern solstice. Depending on the shift of the calendar, the summer solstice occurs some time between December 20 and December 23 each year in the Southern Hemisphere and between June 20 and June 22 in the Northern Hemisphere in reference to UTC.

Though the summer solstice is an instant in time, the term is also colloquially used like Midsummer to refer to the day on which it occurs. Except in the polar regions (where daylight is continuous for many months), the day on which the summer solstice occurs is the day of the year with the longest period of daylight.

People often think of Stonenhenge with they think of the solstices. An enigmatic 5000-year-old World Heritage Site, it pre-dates even the pyramids of Egypt. Its stones weigh up to 60 tons.

Though no one has quite figured out exactly why Stonehenge was built, we have determined that it was built to correspond to the changing of the seasons, specifically the solstices.

Stonehenge was constructed in three phases. It has been estimated that the three phases of the construction required more than thirty million hours of labor.

Speculation on the reason it was built range from human sacrifice to astronomy, though the current thinking is that it may have been used as a place to perform burial rites for many different religions. These rites, archaeologists believe, would have occurred at the solstice.

However it was built and whatever its use, it is awe-inspiring.  Not many things are as old or well-preserved as Stonehenge, with such mysterious roots.

What do you think will last for 5,000 years from our society?  Anything?  Maybe this blog about a 5,000 year-old archaeological site?  :-)

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Venus Transits the Sun

We've had quite an interesting couple of weeks, astronomically speaking.  First the solar eclipse at the end of last month, and just Tuesday, Venus transited the sun!

When Venus passes directly between earth and the sun, we see the distant planet as a small dot gliding slowly across the face of the sun. Historically, this rare alignment is how we measured the size of our solar system.
Transits of Venus have a strange pattern of frequency. A transit will not have happened for about 121 ½ years (prior to 2004, the last one was 1882). Then there will be one transit (such as the one in 2004) followed by another transit of Venus eight years later (in the year 2012). Then there will be a span of about 105 ½ years before the next pair of transits occur, again separated by eight years. Then the pattern repeats (121 ½ , 8, 105 ½ , 8).

If Venus and the earth orbited the sun in the same plane as the sun, transits would happen frequently. However, the orbit of Venus is inclined to the orbit of earth, so when Venus passes between the sun and the earth every 1.6 years, Venus usually is a little bit above or a little bit below the sun, invisible in the sun’s glare.

We watched it using a similar method as we used to watch the eclipse - a pinhole projector.  We used binoculars to enhance the image of the sun projected on the paper though.  And it worked!

You can see it as a little black dot on the white spot of the sun.

Enlarged and enhanced, you can see it slightly larger.

Did you miss this one?  Are you looking forward to the next transit? Mark your calendars - you will be able to see it in 2017, in about 38536 days.

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