A Quick Post on the Summer Solstice

.....I am really trying to keep to a Monday/Thursday posting schedule, largely because hits really go down on the weekend (I hope that no one's bosses read my blog), but since today is the Summer Solstice in the northern hemisphere, here is a special posting.  (I also hope that this is not causing anyone to skip my post on viewing Mercury, posted yesterday.)

.....Here's what set me off.  I was directed to a story on the solstice.  First I'll give you a post on why this is the summer solstice for the northern hemisphere, and try and keep from going off on the problems I saw in the "news" story.  One of the goals for this particular blog is to help people who are teaching astronomy, and teaching the seasons is one of the hardest things to get across to students.  If you are at a cookout, the closer you are to the fire, the warmer you are.  If you are getting and English muffin out of the toaster, the closer you are to the toaster, the warmer your hand is.  In that case, it seems intuitive that the Earth is warmer when we are closer to the Sun.  It seems intuitive, but it is completely, totally, and utterly wrong.

.....The Sun does not move around the Earth (something that is only moderately difficult to get students to accept), but it looks like it does, in the same way that the sky can be treated as a big imaginary sphere.  As the Earth rotates, the North Celestial Pole (NCP) is the point in the sky that appears directly above the Earth's north pole, and the South Celestial Pole and Celestial Equator are ... I hope that you get the idea.  Since the NCP is directly overhead at the north pole, or 90° above the horizon, and the NCP is on the horizon, or at an angle of 0° above the horizon, we can see that the angular height of the NCP is the same as our latitude.  We can then track the apparent path of the Sun across the sky as the ecliptic.  Why we have seasons comes from the fact (a fact that does not depend on whether USA Today says that it is true or not) that the Earth does not rotate with its equator in the same plane in which it is orbiting, but is tilted by 23.5°.  This means that the ecliptic is tilted by 23.5° to the celestial equator (the Earth's equator projected into space) as shown.  The Earth keeps this tilt constant as it orbits, the star Polaris always above the Earth's North Pole, as shown below.  (Not to scale.)

.....The effect of this is that during part of the year, the northern hemisphere is "leaning into" the Sun, and during part of the year, the northern hemisphere is "leaning away" from the Sun.  Imagine that we pick a point on the Earth, and then trace the path that it takes over one day, a circle inscribed across the Earth at your latitude.  The effect that this has on the day comes from two things.  First, notice that the point, at the northern hemisphere summer solstice, spend the majority of its time on the sunlit side of the Earth.  It is hotter in the summer partly because the Sun is above the horizon for a longer period of time.  The northern hemisphere has its longest time of daylight on the summer solstice because the tilt of the Earth points the northern hemisphere most directly towards the Sun on this day. 

 .....The second reason why summer is hotter has to do with the height of the Sun in the sky.  In summer, a quick glance at the diagrams above should show that the Sun is much higher in the sky in the summer, as opposed to the winter.  The Sun provides the same amount of energy to the Earth at all times, but if the Sun is low in the sky that same energy is spread over a greater area, having a lesser effect.  To take Winona, Minnesota, (with a latitude of 44°) as an example, the North Star is 44° above the northern horizon, which means that the celestial equator is (90°-44°=) 46° above the southern horizon.  On the summer solstice, the Sun is  above the celestial equator, or a maximum value of 69.5° above the horizon, as opposed to the winter solstice, when the Sun is only 22.5° above the horizon, at best.  These two effects combine to make summer hotter, and, as the observant may have noticed, at all times (not just on the summer solstice), the seasons in the northern hemisphere are the exact opposite as seasons in the southern hemisphere.

.....The Earth is actually closest to the Sun in January.  This might seem strange, but really, the Earth's orbit is so close to being a perfect circle that this difference is not detectable.  One might expect the Southern Hemisphere's summer to be slightly hotter than the Northern Hemisphere's Summer, but the Southern Hemisphere is mostly water, and water has a strong moderating influence on temperature changes.