The beautiful Fulton Center opened last fall and was an immediate hit with commuters thanks to its soaring glass exterior and the gorgeous, extremely Instagrammable oculus that lets in abundant sunlight.

The MTA wanted to bring as much light as possible into the new Fulton Center, which links nine different subway lines—2, 3, 4, 5, A, C, J, Z and R—in one place. Arup, the lead consultant on the project, started working on a design concept back in 2001, and, with architecture firm Grimshaw, finally settled on the ambitious 110 feet high and 53 feet wide oculus.

The oculus's interior is lined with the dazzling Sky Reflector Net, designed by James Carpenter Design Associates, which helps diffuse the sunlight. There are also "parasols," or reflector panels, at the top of the oculus, which help create sparkle on the inside. Each parasol is tuned to a different sun angle, to pick up the sun at different days and times.

The station was bright and wonderful when it opened in November, but it's only going to get better for the next six months. Arup studied light patterns at the location around the clock for a year, and says tomorrow's Spring Equinox (March 20th) will be special. "The first day of spring, when the sun crosses the equator moving northward... more direct daylight, in terms of intensity and duration, enters the building through the oculus." Here's computer animation of what that will look like:

Arup also provided these details about what to expect for the Summer and Winter Solstices and the Autumnal Equinox:

June 21, Summer Solstice

: "The first day of summer, and the day with the longest amount of time between sunrise and sunset. At noon, the lower floors of Fulton Center will receive the greatest and longest amount of direct sunlight of the year."

September 22, Autumnal Equinox: "The first day of autumn, when the sun crosses the equator moving southward. Each successive day, the circular projection of direct sunlight through the oculus will retreat high up into the interior of the oculus.

December 22, Winter Solstice: The first day of winter, when the sun is farthest south, and the length of time between sunrise and sunset is the shortest of the year. Fulton Center receives the least amount of direct sunlight of the year. Around this time, evening commuters will experience the space as illuminated by interior, architectural lighting, which provides a contrast to illumination by sunlight.

So, while you plan a pagan flash mob for June 21, what is tomorrow's Instagram is going to be—#oculusequinox?

Reporting from Maud Rozee