The reasons for seasons: Why summer just ended and fall began

Yesterday, at exactly 4:44 p.m., the first day of fall began. We all know what that means: pumpkin lattes, the crunch of leaves beneath your boots and brisk weather. But there's more to fall than apple picking -- the new season marks a shift in the Earth's axis.

The celestial event called an "equinox" marks the time when the sun is directly in line with the Earth’s celestial equator. The event occurs twice a year, once in March (for spring) and again in September. During an equinox, day and night last approximately the same amount of time.

"The seasons all have to do with the Earth's axis," Michelle Thaller, assistant director for science communication and higher education at NASA, told "When you think about the Earth rotating around the pole, the rod that goes right through the Earth is not straight but tilted about 23 degrees. That means that for six months the North Pole is tilted more towards the sun and for another six months the South Pole is more tilted."

The Earth spins first on that polar axis once every 24 hours, causing what we know as day and night. Then it moves around the sun once every 365.25 days, a period of time in which we experience four seasons.

The Earth's pole is tilted at 23.4 degrees and always points in the same direction. Twice a year, the sun crosses the celestial equator moving between the Northern and Southern Hemispheres -- marking a change in the sun's rays on Earth's surface. That's why South Americans and fellow Southern Hemisphere inhabitants celebrate Christmas in the summer.

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Now that the autumnal equinox has passed, the Southern Hemisphere is receiving more sunlight than the northern half. Summer in the north has ended and fall has arrived. Winter is over in the south, and spring is just beginning.

The sun will continue to shine southward for three months until December 21st when it reaches the "solstice," a time where the days are shorter and the nights longer.

However, winter does not mean that we are farther away from the sun. Scientifically speaking, the summer is when the sun is higher in the sky and the winter is when the sun is lower in the sky.

"A common misconception is people think we're closer to the sun in the summer and farther away in the winter -- that's not true," Thaller explained. "We're actually closest in December."

The location of the sun affects more than just the seasons, Thaller explains. Astrological or Zodiac signs are determined by the sun's location during the time a person is born.

For example, if a person was born on September 23, they would be considered a Libra. But Thaller says this may not be true.

"Things have shifted so the dates that say 'between these times you are a Leo or a Virgo' may not be true," Thaller told "You have to look at a star chart and find the location of the sun on the day you were born to find our your accurate sign."

Thaller also warns that while we are enjoying the beginning of fall in September, that may not always be the case.

"The seasons will actually switch," Thaller said. "The tilt in the Earth's axis actually turns around once every 26,000 years [causing months to switch seasons]."