|Photo from here.|
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
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).
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,
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
|Honolulu SkyGate at Lahaina Noon. Photo from here.|
|Information from the Bishop Museum.|
Kau Ka La I Ka Lolo
|Map from NOAA.|
|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 solaron the days following the first Lahaina day?
Why are there two Lahainadays for all of the Islands South of Mokumanamana (Necker) island?
Have the islands located north of Mokumanamana ever experienced the Lahainasun?