At 110 stories, the World Trade Center twin towers were tall enough for the Sun to set nearly two minutes later for people on the top floors than for people on the bottom floors.

City of Stars article: Sunset on the World Trade Center

As always, but especially today, keep looking up.

-NDTyson

Nearly everyone I know receives annual e-mails about Mars from an anonymous source, but sent to them by friends who could not resist forwarding the message to their entire address book. The e-mail declares that at the end of August, the planet Mars will sit closer to Earth than it has in the past 60,000 years, thereby offering spectacular views of the Red Planet. The commentary proclaims, with liberal use of exclamation marks, that Mars will appear as bright as (or as large as) the full Moon in the night sky.

This Martian hyperbole dates from August 2003, when the message was mildly factual, but vastly over-stated, leading people to believe Mars would be so bright that you might need sunglasses at night while driving. The rapid spread of this information was like some sort of brain info-virus, and led to at least one daily newspaper comic that showed Mars crashing into a home while the husband and wife were indoors, debating how close the planet will come.

Every 26 months, or so, Earth makes a close approach to Mars, as our smaller, swifter orbit overtakes Mars around the Sun. Because both the orbits of Mars and Earth are mildly elliptical, some close approaches between the two planets are closer than others, but by barely perceptible amounts.

So the proximity of Mars to Earth in August 2003, while indeed closer than in the past 60,000 years, was nonetheless no more meaningful than me swimming a hundred yards out from the California coast (instead of my usual seventy yards) and then declaring to the world I have never been this close to China before.

During close approaches, Mars slowly becomes one of the brightest objects in the night sky. But how bright is that? Slightly brighter than Jupiter's average brightness. And not as bright as that of Venus. Yet nobody has ever issued warning statements about the visibility of Jupiter or Venus. In any case, Mars has had a close approach 3,000 times in recorded history, and, of course, billions of times in Earth's history.

Now it's time for you to send this antidote to ail the infected people in your address book.

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.