The mercury's been rising these past few months across the United States as the region heads into summer, which officially starts on June 21 with the summer solstice. The reason for the season, some might think (and not illogically so), is that according to a natural cycle, the Earth has temporarily shifted closer to the sun and is sweltering in the extra heat.
But this explanation is bogus. In fact, in its oval-shaped orbit around the sun, the Earth will be at its annually farthest point from its star, a position called aphelion, on July 5. Perihelion, the closest point, happened back on Jan. 2. Instead of corresponding to the coldest and warmest days of the year for Americans, it's nearly the opposite. So what gives?
Rather than distance from the sun, the true driver of the seasons on our planet is the tilt of the Earth's axis. Essentially, the Earth does not sit up perfectly straight as it spins; the planet has a slight "lean" as it rotates relative to the plane of its orbit. (Infographic: What Causes Earth's Seasons?)
Over the course of a year, this axial tilt causes the planet's northern and southern halves, or hemispheres, to bask in different levels of sunlight, which in turn sends temperatures spiking or plummeting.
"As we orbit around the sun, the tilt of the Earth relative to that orbit ends up leading to dramatic seasonal changes in the amount of energy we receive from the sun," said Dan Barrie, program manager at the National Oceanic and Atmospheric Administration's Climate Program Office. "As a result, the temperatures we end up experiencing are radically different."
Scientists think the Earth got an initial axial tilt from collisions with myriad objects pinballing around during the early solar system's formation 4.5 billion years ago.
Many factors, such as gravitational interactions with the moon, sun and planets, continue to alter the Earth's tilt by a few degrees on long-term scales of tens of thousands of years. The planet's current axial tilt — that is, the amount it deviates from a perpendicular, zero degrees relative to the plane of its orbit — is 23.44 degrees, and on the decline.
To visualize the Earth's axial tilt, think of the globe as a toy top that wobbles as it whirls on a surface. "As the Earth rotates around the sun, it's not an upright top — it spins around a little bit," said Barrie.