This weekend, and the second weekend in July, "Manhattanhenge" once again returns to New York City. Images, my full write-up, times, and other details are posted on our Manhattanhenge page.

As you may know, Manhattanhenge takes place on two consecutive days, twice a year, when the setting Sun aligns precisely with the Manhattan street grid, creating radiant sunsets that burst across our brick and steel canyons, simultaneously illuminating both the north and south sides of every street. A rare and beautiful sight.

Unnoticed by many, the sunset point actually creeps day to day along the horizon: northward until the first day of summer, then returning southward until the first day of winter. In spite of what pop-culture tells you, the Sun rises due east and sets due west only twice per year - the first day of spring and of autumn. Every other day, the Sun rises and sets elsewhere on the horizon.

While we are on the subject, when viewed from all latitudes north of the Tropic of Cancer (23.5 degrees north latitude), the Sun always rises at an angle up and to the right, and sets and an angle down and to the right. That's how you can spot a faked sunrise in a movie: it moves up and to the left. Filmmakers are not typically awake in the morning hours to film an actual sunrise, so they film a sunset instead, and then time-reverse it, thinking nobody will notice.

The consecutive days for Manhattanhenge identify when half the Sun's disc and when the entire Sun's disc is setting on the grid. If the Sun were to set perpendicular to the horizon then these would be the same day. But since the Sun's path angles down and to the right, and shifts slightly from one day to the next, we get two days of Manhattanhenge, a half-sun sunset (my personal favorite for photographs) and full-sun sunset.

Astronomers have spotted a rare, complex cosmic collision of four galaxy clusters. A spectacular composite image from NASA's Chandra X-Ray Observatory and Hubble Space Telescope revealed the pileup—and its likely source.

To learn about other recent astronomical discoveries, visit the Science Bulletins website.

Should a rocket blast off on schedule early Tuesday evening, May 5, 2009, from NASA's Wallops Island Flight Facility in Virginia, a potentially spectacular sight might be visible across a wide swath of the U.S. Eastern Seaboard.

It would be only the fourth attempt at launching an orbital rocket from this coastal Virginia range—located just south of Assateague Island—in the last 13 and a half years. The first time NASA attempted an orbital launch from Wallops, in October 1995, the liftoff of a 50-foot-tall Conestoga rocket ignited normally, but the vehicle exploded over the Atlantic just 46-seconds later. A problem with the rocket’s guidance system was blamed.

Then in December 2006, a 69-foot, 5-foot wide, 35-ton, four-stage Minotaur I rocket successfully launched the TacSat-2 satellite, carrying a semisecret payload from the Pentagon's Missile Defense Agency. In April 2007, a Minotaur rocket sent the NFIRE satellite into orbit.

Now another Minotaur I rocket awaits liftoff from the Wallops Flight Facility next Tuesday evening, no earlier than 8:00 p.m. EDT. The chief goal of this flight is to place the 880-pound TacSat-3 satellite with its trio of payloads that will offer real-time imagery (within 10-minutes of collection), sea-based information transmitted from ocean buoys and plug-and-play avionics to assist warfighters in keeping one step ahead of their adversaries. In addition, three cubesats will be launched as secondary payloads on the TacSat-3 mission.

A launch window from May 5 to 9, from 8:00 p.m. to 11:00 p.m. EDT each day, has been established to take into account bad weather or equipment glitches (see Final Points below). A launch after 8:00 p.m. EDT would occur just after sunset along the entire Atlantic Coastline.

What to expect

Over the years, similar rocket firings have routinely taken place from California's Vandenberg Air Force Base. Rocket launches that have occurred around the time of sunrise or sunset have left long, glowing contrails in their wake that have been seen for a few hundred miles across the Desert Southwest; often becoming contorted by high level winds into strange and exotic patterns and sometimes, prismatic colors.

While many Westerners are fairly familiar with such sightings, they are all but unknown here in the East and as such may end up surprising millions of people should the Minotaur I lift off on schedule at 8:00 p.m. EDT or shortly thereafter on Tuesday evening.

Based on a very similar launch from Wallops Island in December 2006 and similar dusk and dawn launches from Vandenberg as a guide, I've determined that it should be possible that Tuesday's post-sunset launch may be visible as far north as southern Maine; as far south as northeastern Florida and as far west as eastern Kentucky. The rocket will be launched on a southeast trajectory. Approximately six minutes after launch it will be passing north of Bermuda. Three minutes later it will reach orbital altitude over the middle of the North Atlantic.

Observers who are situated within about 800 statute miles of the Wallops Island Flight Facility appear to have a reasonable chance of catching a view of the Minotaur I contrail within the first few minutes after launch.

The key to making a sighting is to have a clear, unobstructed view of the horizon in the direction of Wallops Island. For example, a viewer in Raleigh, North Carolina should look toward the northeast; in Boston, Massachusetts look southwest; in Wheeling, West Virginia it will be due east.

Areas farther to the northeast (toward southern New England) have an advantage since skies will be darker—sunset will come somewhat earlier than it will along the Mid-Atlantic Coast. At Wallops, it's at 7:57 p.m., but from Boston it's at 7:49 p.m.

Farther to the west, in the Ohio Valley, the Sun will still be above the horizon so the launch may only be barely visible, if at all against the blue daytime sky. But should the launch be delayed by just 30-minutes, sunset will arrive, sufficiently darkening local skies.

Final points

The Minotaur I is a launch vehicle sometimes called half man and half beast because it combines features of the Minuteman missiles and Pegasus rockets. It also merges space technologies designed for both military and commercial ventures.

Another factor in this launch is the upcoming May 11 launch of the Space Shuttle Atlantis at Cape Canaveral, Florida. According to Wallops Public Affairs officer Keith Koehler, the Wallops tracking systems are needed to support a shuttle Launch. We have to stand down because of the Space Shuttle launch on May 11. We have assets that support the launch. Then I believe there is another launch at the Cape that will be using assets that we use.

Wallops is NASA's cynosure for tracking orbital payloads and receiving meteorological information relating to North America. And although it has never been used for this purpose, Wallops can also boast that its 8,750-foot runway, usually reserved for winged-aircraft safety tests, is an approved shuttle emergency-landing site.

You can find more information on the launch, including the latest launch status and contact information, a webcast of the launch, and the countdown status on Twitter.