Author Archives: Katie Curtis

How large an asteroid could a person jump off?

How large an asteroid could a person jump off? This interesting question goes back to 1952 or even earlier. It turns out that the asteroid must be no bigger than 3.9 v(ρe / ρ) kilometers in diameter, where ρ is the asteroid’s density and ρe is that of Earth in the same units (S&T:...

When you state that a telescope drive has a periodic error of 10 arcseconds, does that mean that it has a tracking accuracy of ±10 arcseconds or ±5 arcseconds?

In your product reviews, when you state that a telescope drive has a periodic error of, say, 10 arcseconds, do you mean that it has a tracking accuracy of ±10 arcseconds or ±5 arcseconds? All gear systems have inherent mechanical errors that limit the accuracy with which a telescope drive can track the sky’s...

Is it possible to see the crescent of Venus?

Is it possible, with better than normal eyesight, to see the crescent of Venus? That question has been controversial, but in fact some people can. The rough rule of thumb is that someone with excellent vision can just resolve two image elements 60 arcseconds (60") apart. At times, this is enough resolution to make...

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)....

What happened to the “spokes” in Saturn’s rings?

In 1979 the Voyager spacecraft revealed “spokes” in Saturn’s rings. Yet recent images from Cassini have shown no spokes. What happened to them? Those spokes have kept planetary scientists scratching their heads for decades. It’s thought that the spokes are radial fingers of dark, extremely fine dust particles that become electrostatically charged by pulses...

When the Moon misses the Sun by the greatest amount north (or south), could the lunar crescent be seen in a telescope at new Moon?

Is the Moon’s orbit inclined sufficiently that, when it misses the Sun by the greatest amount north (or south), the lunar crescent could be seen in a telescope at new Moon? Probably not. The inclination of the Moon’s orbit to the ecliptic varies from 5.0° to 5.3°. French astronomer André Danjon (1890–1967) showed that...

How is the time of new (or full) Moon defined?

How is the time of new (or full) Moon defined? Astronomically, the Moon is new when it and the Sun have the same celestial longitude. The Moon is at first-quarter phase when its longitude is 90° greater than that of the Sun. The Moon is full when its longitude is 180° greater, and at...

Do Saturn’s moons cast observable shadows on Saturn?

I’ve seen the tiny black shadows cast by Jupiter’s moons. Do Saturn’s moons cast observable shadows on that planet? Shadow transits of Titan, Saturn’s largest moon, have been seen with a telescope as small as 2 7/8-inch (73-millimeter) aperture. In 1891, two English observers saw the shadow of Rhea on Saturn with 6 1/2-inch...

Why do people doing CCD imaging often stack five 1-minute exposures instead of taking just one 5-minute exposure?

Why do people doing CCD imaging often stack, say, five 1-minute exposures instead of taking just one 5-minute exposure? Modern digital cameras capture faint astronomical objects with much shorter exposures than their film-based counterparts did, but it still takes an exposure of many minutes to produce a good picture. So-called image stacking is the...

What would happen if I fed animatter into a black hole?

What would happen if I fed animatter into a black hole? Could this be a treatment to get rid of black holes? No. Antimatter has positive mass just like ordinary matter, so the black hole would merely get larger and heavier. Whatever fireworks happened inside the hole, if the anitmatter met up with ordinary...