Summer Solstice 2013: Longest Day Of Year Begins Thursday, June 20

The longest day of 2013 is finally here -- but this year, it comes with a twist.

While the solstice in the northern hemisphere traditionally falls on June 21 -- and this year it will occur on that date at 1:04 a.m. EDT -- it will begin on Thursday, June 20, for parts of the western U.S., according to the website of the Clark Planetarium. The time of the solstice depends upon your position on Earth and, as a consequence, where you are in relation to the sun.

The summer solstice occurs when Earth's axis is the most tilted toward the sun -- the angle is known as "maximum axial tilt." As a consequence of this specific orientation, the sun rises at its most northeasterly point along the horizon and also sets at its most northwesterly point in the northern hemisphere.

The solstice isn't the only big celestial event this week. Skywatchers are gearing up for the arrival of the 2013 supermoon, which is set to peak June 22-23 and deliver the biggest, brightest moon of the year.

Please send us your supermoon photos this weekend!

You can tweet your photos with hashtag #HPsupermoon. Or, you can submit them directly to our "Supermoon 2013" slideshow, which will be featured in our supermoon live blog on the evening of Sunday, July 23.

We'll be collecting user photos from all over, and yours may be featured!