It has been hot here for the past few months. In fact, the first 100-degree day of the year in Phoenix was May 3. But astronomically, summer just started Saturday.
So, what causes seasons on Earth? It has to do with the tilt of the Earth. If one ran a straight line from the South Pole to the North Pole, it would be tilted at 23.5 degrees.
Because of this tilt and our revolution around the sun, during part of the year, the northern hemisphere is pointed toward the sun. During the other part of the year, it is pointed away from the sun.
What happened Saturday at 3:51 a.m. Arizona time is that the sun was directly overhead and at its furthest northern latitude, which is 23.5 degrees above the equator. This spot is known as the Tropic of Cancer. The event also marked the longest day of the year for the Northern Hemisphere. In Phoenix, our longest day is 14 hours, 22 minutes.
As we continue to revolve around the sun, the location of the sun when it's directly overhead will begin traveling south and will be directly over the equator on Sept. 23. This is called the autumnal equinox; on this day, every location on Earth receives 12 hours of daylight and 12 hours of darkness.
Finally, on Dec. 21, the sun will be at its furthest southern location, which is 23.5 degrees south. This spot is known as the Tropic of Capricorn. That marks the first day of winter in the northern hemisphere. This is also the shortest day of the year in Phoenix; the day lasts 9 hours, 56 minutes.
Another fun fact about the sun and Earth: Did you know that the Earth's path around the sun is not a perfect circle? It is actually elliptical, meaning part of the year the Earth is closer to the sun.
Would you believe that during our summer, Earth is farthest from the sun (94.5 million miles) and during our winter, Earth is actually closer to the sun (91.5 million miles)? Sounds backward, but it shows that the 3-million-mile difference from summer and winter is not enough to substantially heat or cool the planet. It's the tilt of the Earth that drives our seasons.
