Time & Motion

Daylight Hours

No matter where in the world you live, do you get the same number of daylight hours over the course of a year? No. The equator actually gets fewer hours of daylight than most other latitudes. Any given place would be in daylight exactly 50 percent of a year’s time if Earth moved in...

Earliest sunrise of the year

The earliest sunrise of the year doesn't always occur on June 20th (the solstice), which has the longest day. Why? The earliest sunrise actually depends partly on latitude. In 2004, for latitude 40° north, it was June 13th. For 50° north, the earliest sunrise was on June 16th. At 30° south, where the seasons...

Four Galilean moons shadows

Can all four Galilean moons cast their shadows on Jupiter ’s disk at the same time? No. A triple shadow transit is as good as it gets (S&T: July 2004, page 94). The motions of the three inner moons are gravitationally locked so that, as English astronomer George B. Airy wrote more than a...

How can WR 20a, a binary star in Carina, move so fast?

In a News Note (S&T: November 2004, page 16) you described WR 20a, a binary star in Carina with components of 83 and 82 solar masses and an orbital period of 3.686 days. How can two huge balls of gas move so fast? Grab any introductory astronomy textbook and turn to the chapter on...

How do you convert celestial coordinates for equinox 1950.0 to 2000.0?

I use Burnham’s Celestial Handbook, which gives objects’ celestial coordinates for equinox 1950.0. How do I convert these to 2000.0, the current standard? First off, the difference isn’t great. Fifty years of precession change an object’s right ascension and declination by a total of 0.7° at most (if it’s near the ecliptic; less elsewhere)....

SAO numbers for Go To telescope

I want to observe double stars, but my Go To telescope needs their SAO numbers. Where can I get them? SAO numbers come from the Smithsonian Astrophysical Observatory Star Catalog (1966). They aren’t normally used by double-star enthusiasts, who prefer special notations such as ADS 1630, S748, and Krueger 60.Finding equivalent catalog designations can...

When will the last total solar eclipse occur?

With the Moon slowly receding from Earth, there will come a time when it no longer has sufficient angular size to cover the Sun completely. When will the last total solar eclipse occur? When the Moon’s at its closest and biggest and it can no longer cover the Sun at its farthest and smallest,...

Why are Kuiper Belt discoveries made at aphelion?

Why is it that so many of the recent Kuiper Belt discoveries — including the “10th planet,” 2003 UB313 — have been made while the objects are at aphelion? Timing and luck. Objects that orbit the Sun spend most of their time near the farthest point of their orbits, where they move most slowly....