Today marks a significant milepost in our planet's yearly trip around the sun. Known as the summer solstice, today is the longest day of the year and the first day of summer.

Usually, the summer solstice occurs on June 21. But not this year, at least not in the United States, anyway.

According to the U.S. Naval Observatory, this year's summer solstice occurs at precisely 5:57 tonight on the West Coast. That is 12:57 a.m. on June 21 on the prime meridian in Greenwich, England.

Scott Kardel, an astronomer at Palomar Observatory, explained that the day occurs one day earlier this year for the same reason that an extra day is added to calendars every four years.

"It is essentially the same reason we have leap year every four years," Kardel said, noting that a year is 365.24 days long. He said it is that extra one-quarter of a day that caused summer solstice to occur on June 20 this year.

"Events like this that are tied to the sky tend to vary a little bit," Kardel said.

Earth's seasons are caused by its axial tilt.

Kardel said that if the Earth were not tilted 23.5 degrees on its axis, the summer solstice would be just another day.

"Venus has essentially no tilt, so it has no seasons," Kardel said.

Other planets have too much tilt.

"Uranus is tipped almost 90 degrees," he said, adding that it takes about 84 years for the planet to circle the sun.

"It has seasons, too, but each of them are about 21 years long," Kardel said.

The tilt means that the top half of the world receives more sunlight for longer periods in the summer and less in the winter, making August much hotter than January.

The Earth's axial tilt also affects our perception of sunrise and sunset. Depending on whether it's summer, winter or somewhere in between, the sun appears to set in the north, south or due west. Summer solstice offers the northernmost sunset of the year. Starting Monday, the sun will begin to creep slowly southward as the Earth continues its path around the sun.

Mike Fowler, co-owner of Oceanside Photo and Telescope on Mission Avenue, said he has always been fascinated with the summer solstice.

He noted that mankind's understanding of the summer and winter solstices, and vernal and autumnal equinoxes at the beginning of spring and fall, has led man to build such artifacts as the famous Stonehenge in southern England, which astronomers now think was created as a kind of astronomical calculator that could be used to predict celestial events.

"It's interesting how the extremes of nature seem to define human civilization," Fowler said.

It is not only sunshine that is more abundant during summer solstice.

Astronomers also note that full moons occurring close to the solstice are more spectacular because they follow the lowest path across the sky. The path means the full moon passes closer to the horizon, making it appear much larger and brighter than usual.

Contact staff writer Paul Sisson at (760) 901-4087 or psisson@nctimes.com.