On Tuesday (Sept. 8, 2009) and Wednesday (Sept. 9, 2009) the New York metro area will have three great opportunities (local weather conditions permitting) to see the Space Shuttle Discovery and the International Space Station flying in tandem across our evening sky.

Currently, Discovery is docked to the Space Station, but at 3:27 p.m. on Tuesday it will undock from the Station and slowly begin to pull away from it.

On Tuesday, at 7:42 p.m., both Discovery and the Space Station will rise above the SSW horizon and will track toward the ENE. They will appear as two bright moving "stars." Discovery will appear to lead the Space Station across the sky. The Space Station, however, should appear as the brighter of the two objects. They will be separated by roughly 5 or 6 degrees; your clenched fist held at arm's length measures roughly 10-degrees, so the Shuttle and Space Station will be separated by about a "half fist" as they move across the sky. At their highest point, they will stand 33-degrees above the SE horizon and at 7:44:30, as an added bonus, they will pass directly above the planet Jupiter. This pass should take 5 minutes 17-seconds from start to finish.

Also on Tuesday evening, a second pass will begin at 9:18 p.m. Discovery and the Space Station will appear a little farther apart. They will rise above the W horizon and will track toward the NNW. After 2 minutes 29 seconds, however, they will rapidly fade out, 25-degrees above the horizon, as they pass into the Earth's shadow.

On Wednesday, comes the third and final double pass. The two space vehicles will be much farther apart ... the Shuttle will lead the ISS by about one minute. Watch for Discovery at 8:05 p.m. rising above the WSW horizon, moving toward the NE; the Space Station will follow along the same track about a minute later. At their highest point, they will stand 64-degrees above the NW horizon. This pass should take 5 minutes 42-seconds from start to finish.

Discovery is scheduled to return to the Kennedy Space Center in Florida on Thursday (Sept. 10) at 7:06 p.m.

If you go out and carefully study the sky near dusk or dawn, and you have relatively dark skies, the odds are that you should not have to wait more than 15 minutes before you see one of the nearly 15,000 satellites now in orbit around Earth. Most of these "satellites" are actually just "space junk" ranging in size from as large as 30 feet, down to about the size of a softball. The Joint Space Operations Center (JSpOC) headquartered at Vandenberg AFB in California, keeps a constant watch on all orbiting debris. And in fact most satellites—especially the bits of debris—are too faint to be seen with the unaided eye. But depending on who's counting, several hundred can be spotted with the unaided eye. These are the satellites that are large enough (typically more than 20 feet in length) and low enough (100 to 400 miles above Earth) to be most readily seen a sunlight reflects off them.

The biggest

The International Space Station (ISS) is by far the biggest and brightest of all the man-made objects orbiting the Earth. On-orbit construction of the station began in 1998, and is scheduled to be complete by 2011, with operations continuing until around 2015. More than four times as large as the defunct Russian Mir space station, the completed International Space Station will ultimately have a mass of about 1,040,000 pounds (520 tons) and will measure 356 feet across and 290 feet long, with almost an acre of solar panels to provide electrical power to six state-of-the-art laboratories. Presently circling the Earth at an average altitude of 216 mi (348 km) and at a speed of 17,200 mi (27,700 km) per hour, it completes 15.7 orbits per day and it can appear to move as fast as a high-flying jet airliner, sometimes taking about four to five minutes to cross the sky. Because of its size and configuration of highly reflective solar panels, the space station is now, by far, the brightest man-made object currently in orbit around the Earth. On favorable passes, it can appear as bright as the planet Venus, at magnitude -4.5, and some 16 times brighter than Sirius, the brightest star in the night sky. Some have made estimates as bright as magnitude -5 or -6 for the station (smaller numbers represent brighter objects on this astronomers scale). And as a bonus, sunlight glinting directly off the solar panels can sometimes make the ISS appear to briefly "flare" in brilliance to as bright as magnitude -8; more than 16 times brighter than Venus!

Other things to see

Along with the ISS, you can also look for a space shuttle on those missions when one is approaching or departing from the space station. Also visible to the naked eye is the newly refurbished Hubble Space Telescope. And with good binoculars, you might also try sighting the infamous "ISS Toolbag" which accidentally was allowed to drift into space when veteran spacewalker and astronaut Heide Stefanyshyn-Piper lost her grip on the backpack-sized bag last Nov. 18

Viewing opportunities

From now through the final days of July, North Americans and Europeans will have many opportunities to see the ISS flying over their homes, due chiefly to a seasonal circumstance. Nights are now the shortest and the time that a satellite in a low-Earth-orbit (like the ISS) can remain illuminated by the Sun can extend throughout the night, a situation that can never be attained during other times of the year. Because the ISS circles the Earth about every 90 minutes on average, this means that it's possible to see it not just on one singular pass, but for several consecutive passes. Moreover, because the ISS revolves around the Earth in an orbit that is inclined 51.6-degrees to the equator, there are two types of passes that are visible. In the first case (we'll call it a "Type I" pass), the ISS initially appears over toward the southwestern part of the sky and then sweeps over toward the northeast. About seven or eight hours later, it becomes possible to see a second type of pass (we'll call it "Type II"), but this time with the ISS initially appearing over toward the northwestern part of the sky and sweeping over toward the southeast. During these next several weeks, Type I passes will initially be visible in the morning hours, prior to sunrise. By early July, Type I passes will be visible during the evening hours, just after sunset, while Type II passes will be occurring in the early morning. By late July, visibility of the Type II passes will have shifted into the evening hours. FROM NEW YORK CITY the best ISS passes in July will be Type II. If you are an early riser, there will be two excellent dawn passes of the Space Station. In both cases, it will trace a high northwest to southeast path across the twilight sky. The first opportunity comes on July 6 from 4:43 to 4:48 a.m. There will be another, very similar pass two mornings later on July 8 between 3:56 and 4:02 a.m. If these times are too early for you, then just wait a couple of weeks and the ISS will make two similarly high NW to SE passes during the evening hours. On July 23, it will be visible between 9:44 and 9:48 p.m. and two evenings later you'll see it moving along the same NW to SE path between 8:58 and 9:03 p.m. Of course there will be many other opportunities to see the ISS during July, but at lower altitudes and for shorter durations.

When and where to look

So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these four popular web sites: Chris Peat's Heavens Above Science@NASA's J-Pass NASA's SkyWatch Spaceweather.com Each will ask for your zip code or city, and respond with a list of suggested spotting times. Predictions computed a few days ahead of time are usually accurate within a few minutes. However, they can change due to the slow decay of the space station's orbit and periodic reboosts to higher altitudes. Check frequently for updates. Another great site is this one: http://www.n2yo.com/?s=25544 Which provides real-time satellite tracking and shows you at any given moment during the day or night over what part of the Earth the ISS happens to be.

Viewing tips

Some passes are superior to others. If the ISS is not predicted to get much higher than 20-degrees above your local horizon, odds are that it will not get much brighter than second or third magnitude (10-degrees is roughly equal to the width of your fist held at arm's length). In addition, with such low passes, the ISS will likely be visible for only a minute or two. Conversely, those passes that are higher in the sky—especially those above 45-degrees—will last longer and will be noticeably brighter. The very best viewing circumstances are those that take the ISS on a high arc across the sky about 45 to 60 minutes after sunset, or 45 to 60 minutes before sunrise. In such cases, you'll have it in your sky upwards to four or five minutes; it will likely get very bright and there will be little or no chance of it encountering the Earth's shadow. While the ISS looks like a moving star to the unaided eye, those who have been able to train a telescope on it have actually been able to detect its T-shape as it has whizzed across their field of view. Some have actually been able to track the ISS with their scope by moving it along the projected path. Those who have gotten a good glimpse describe the body of the Space Station as a brilliant white, while the solar panels appear a coppery red. For evening passes, the ISS will usually start out rather dim and then tend to grow in brightness as it moves across the sky. In contrast, for the morning passes, the ISS will already be quite bright when it first appears and will tend to fade somewhat toward the end of its predicted pass. This is due to the change in the angle of sunlight hitting the vehicle. Lastly, remember that in certain cases, the ISS will either quickly disappear when it slips into the Earth's shadow (during evening passes) or quite suddenly appear when it slips out of the Earth's shadow (during morning passes). This becomes increasingly more likely for passes that take place more than 90 minutes after sunset or more than 90 minutes before sunrise.

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.