Friday, June 21, 2013

The Solstice is Upon Us!

Photo from here.
Today is June 21st, the longest day of the year in Hawai'i.  Today the sun came up at 5.50am and will set at 7.16pm, for a total day length of 13 hours, 25 minutes, and 54 seconds.  We call this day the June (or Summer) Solstice.  After today the days will continue to get shorter until December 21st, with a day length of 10 hours, 50 minutes, and 12 seconds.  December 21st is the December (or Winter) Solstice.  In this post we will describe why there is variation in day length over the course of the year.

The Earth-Sun Relationship...

As we all know, the Earth revolves around the sun, a journey that takes approximately 365.22 days.  We also know that the earth is tilted on its axis at an angle of approximately 23.5 degrees.  This is why virtually every globe you ever see is tilted; it is demonstrating the earth's true orientation towards the sun.  But did you know that the tilt is always in the same direction?  This characteristic of the earth's orbit is called axial parallelism, and it is why days are shorter in the winter and longer in the summer.  This in turn is one of the biggest factors in seasonal variability; it is why the continental United States experiences winter, spring, summer, and fall.

Diagram from here.  

Have a look at the model we've provided.  As you can see, no matter what time of year it is, the earth's tilt is in the same direction.  If it is December, then more of the southern hemisphere is exposed to the sun at any given time, and less of the northern is bathed in glory of the sun's warming touch.  It follows then that in December it is summer in the southern hemisphere, and winter in the northern hemisphere.  If you imagine with your mind's eye that the earth is spinning around its axis (remember, one rotation equals one day), you can see that since more of the southern hemisphere is in the sun, the days are longer.  The opposite is true with the northern hemisphere.  Now look at the earth when it is June and the planet's northern hemisphere is at its maximum tilt towards the sun.  Can you see that more of the northern hemisphere is exposed to the sun, whereas less of the hemisphere is?  Thus the days are no longer in the northern hemisphere than in the southern.  Now look at the north pole.  Again, imagine with your mind's eye that the earth is spinning on its axis.  Look at the places close to the north pole.  Is there ever a point during a day (one complete rotation) that these points enter the darkness?  If you answered "no" you are correct!  These areas experience 24 hour days at this point, whereas at the south pole and near it there are 24 hour nights!

So at this point it should make sense to you that there is one day during the year when the northern hemisphere is at its maximum tilt towards the sun (whereas the southern hemisphere is a it maximum point away from the sun), and another point, roughly half a year later where the northern hemisphere is at its maximum tilt away from the sun (whereas at this point the southern hemisphere would be pointed towards the sun).  These two days are called the solstices, and they are the longest and shortest days of the year respectively in the northern hemisphere (and the opposite in the southern hemisphere).  

Now look back at the diagram.  There are two days of the year, one in March and the other in September, where the earth is pointed neither towards nor away from the sun; rather the tilt of the earth is perpendicular to a line drawn from the sun to the earth.  On these days every part of the earth receives 12 hours of daylight and 12 hours of night.  These days are called equinoxes.

So What Are the Tropics?

We've all heard the term "tropics", as in tropical storm, tropical paradise, and tropical fish.  But what does this really mean?  The "tropics" describes a very specific area on the earth's surface: all latitudes where the sun passes directly overhead at some point during the year.  Let's go back to our diagram of the earth-sun relationship.  Since the earth is rounded, there is a point on the earth's surface that is closer to the sun than all other points.  If you were standing on that point, the sun would be directly overhead.  Now since the earth is tilted, the spot on the earth where the sun is directly overhead changes over the course of the year.  The spot where the sun is currently overhead is called the subsolar point, and the latitude where the sun is directly overhead is called the solar declination.  Since the earth is tilted at an angle of 23.5 degrees, the subsolar point is found between 23.5 north latitude (the Tropic of Cancer) and 23.5 south latitude (the Tropic of Capricorn).  The area between these two lines of latitude is the tropics, and as we all know, Hawai'i is in the tropics.  This means that the sun will be directly overhead at solar noon in Hawai'i on two days during the year, one in May, and one in July.  Any place outside the tropics never ever experiences the sun directly overhead!  This is one more aspect of Hawai'i's geography that makes it special.  

Lahaina Noon?

Honolulu SkyGate at Lahaina Noon.  Photo from here.  
If you've ever been to Maui (or if you live there!) you may have visited Lahaina.  Lahaina is a major tourist attraction and is known as an old capital of the Hawaiian kingdom.  But Lahaina also gives its name to the day when the sun is directly overhead at solar noon in Hawai'i.  We call this "Lahaina Noon".  This name was immortalized in a contest sponsored by the Bishop Museum.  "Lahaina Noon" was chosen for these two days when the sun is at its greatest intensity because La Haina means "cruel sun" in Hawaiian.  On this day at solar noon you can witness something that no one on the continent ever sees (at least on clear days): a complete lack of shadows.  Since the Hawaiian Islands run to the northwest, Lahaina Noon happens on different days at different places.  In the table below you can see the dates for this year (2013); the Bishop Museum normally provides the dates on their website as well.

Information from the Bishop Museum.

Kau Ka La I Ka Lolo

Map from NOAA.  
The old Hawaiians were very attuned to the movement of celestial bodies (the sun, stars, and the moon) and referred to the day when the sun was directly overhead as kau ka la i ka lolo.  This expression can be translated as "the sun rests on the brains".  We understand now that these days had special significance to the old Hawaiians as well.  Based on research by University of Hawai'i Department of Anthropology graduate Dr. Kekuewa Kikiloi we have learned that many of the archaeological remains on Mokumanamana Island (generally known as Necker Island, one of the Northwest Hawaiian Islands) are tied to rituals associated with the passing of the sun directly overhead.  Mokumanamana happens to be right on the Tropic of Cancer, and so the sun is directly overhead here.  The Old Hawaiians were keenly aware of this fact, and so the island had a priestly significance to them; high-ranking priests would make periodic journeys to Mokumanamana to calibrate their calendars and for other ceremonies.

Necker island landsat image from Papahanaumokuakea National Monument website.  


Will the islands receive more energy from the sun on there respective Lahaina days or on the June Solstice? 

What compass direction will the Sun be oriented at solar noon on the days following the first Lahaina noon day? 

Why are there two Lahaina noon days for all of the Islands South of Mokumanamana (Necker) island?

Have the islands located north of Mokumanamana ever experienced the Lahaina noon sun?

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