The phenomenon, nicknamed "Manhattanhenge," happens four times a year — twice with a full sunset and twice with a half sunset. (Last night's sunset show was rained out.) Tonight's event will be the fourth and last of the year; the city previously experienced Manhattanhenge on May 28, May 29, and most recently yesterday (July 12).

Current weather forecasts for New York City show a 30 percent chance of rain at sunset this evening, but if conditions are clear, the full effect of Manhattanhenge will be visible at 8:24 p.m. EDT. At that time, the half sunset will be at its optimal point, when half of the sun appears on the horizon, and the other half below. Warning: Never look directly at the sun, as it is very dangerous and can cause permanent eye damage. [See Photos of Amazing Sunsets & Sunrises]

For the best experience, head outside about a half-hour earlier than the time of the actual sunset, said astrophysicist Neil deGrasse Tyson, director of the American Museum of Natural History's Hayden Planetarium, in a blog post on the museum's website.

"For the best effect, position yourself as far east in Manhattan as possible," wrote Tyson, who coined the term "Manhattanhenge" in 2001, and is credited with popularizing the event. "But ensure that when you look west across the avenues you can still see New Jersey."

Some of the clearest cross streets include 14th, 23rd, 34th, 42nd, 57th and several of the streets immediately adjacent to these wide boulevards. "The Empire State building and the Chrysler building render 34th street and 42nd streets especially striking vistas," Tyson said.

Manhattanhenge is a play on the name Stonhenge, a prehistoric monument that lies north of the modern-day city of Salisbury, England. Once a year, the sunrise is perfectly aligned with the stones that make up the mysterious megalith.

Several other cities in the United States with grid-patterned streets also experience similar sunsets, including Baltimore and Chicago.

Editor's note: If you snap an amazing photo of Manhattanhenge and you'd like to share it for a possible story or image gallery on , please send images and comments, including equipment used, to Denise Chow at dchow.

Follow Denise Chow on Twitter @denisechow. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.