Welcome to Houston Astronomical Society

by Will Sager

Meteor observers were recently excited about the possibility of an out-of-nowhere meteor shower, called the Tau-Herculids, that offered the possibility of a “storm” at the end of May. Many years ago, my uncle described seeing the 1966 Leonid meteor storm in Virginia and said meteors were falling too fast to count all over the sky. Later I read in Sky and Telescope about observers at Kitt Peak, who got the best display. They witnessed a rain of meteors estimated at 10-40 per second. I have been observing meteors for 45 years and have never seen such a storm. The closest I came to it was the 2001 Leonid shower, when the Earth passed through a rich stream of debris from comet Temple-Tuttle. That night, under dark skies in Arizona, I logged over 1200 over the course of the night. It was awesome, but not quite the storm that my uncle saw. Would the Tau Herculids be my storm?

Meteor showers arise from debris shed by comets or asteroids. The debris expands along the parent body orbit and if the Earth encounters that debris stream in its yearly swing around the Sun, there will be a meteor shower. Owing to perspective, the meteors seem to radiate from a single spot in the sky – like the like the snowflakes when you drive into a snowstorm (Texans will have to take my word on this). The shower is named by the location of this radiant. The Leonids from from Leo, the Perseids from Perseus, and the Tau-Herculids from a point near the star tau-Hercules. Most meteors are tiny grains, dust to sand size, and they cause streaks because they run into the atmosphere at astonishing speeds (more than 50 kilometers per second) and this causes impacted upper atmosphere atoms to be ionized and glow briefly. Every now and then, a larger fragment plows into the atmosphere, causing a bolide (aka fireball), which is an exceptionally bright meteor (usually causing the observer to hoot in delight). The most reliable annual meteor showers have debris all along the parent body orbital path, which happens to intersect the Earth’s orbit. Every year, the Earth runs through the stream, producing the annual shower.

by Jim King

Charles Messier, in his active observation life, used several different telescopes.

Here follows a list of telescopes Messier listed and stated he used in 1765-69, published in the Connaissance des Tems for 1807 (here taken from Kenneth Glyn Jones' book; FL means "Focal Length", "Mag." Magnification, unfortunately he normally doesn't give the aperture):

1. Ordinary refractor of 25 foot FL, Mag. 138x
2. Achromatic refractor, 10.5 foot FL, owned by M. de Courtanvaux, Mag. 120x
3. Achromatic refractor, 3.25 foot FL (Dollond), owned by Duc de Chaulnes, Mag. 120x
4. Ordinary refractor of 23 foot FL, Mag. 102x
5. Ordinary refractor of 30 foot FL, owned by M. Baudouin, Mag. 117x
6. Campani refractor, owned by M. Maraldi, Mag. 64x
7. Gregorian reflector ('Short') 6 feet FL, owned by M. Lemonnier, Mag. 110x
8. Gregorian reflector 30 feet FL, 6 inch aperture, Mag. 104x
9. Newtonian reflector 4.5 foot FL, Mag. 60x
10. Refractor 1 foot FL, 3-inch aperture, owned by M. de Saron, Mag. 44x
11. Refractor 19 foot FL, of the Paris Observatory, Mag. 76x

As there's always a magnification given, it seems that the idea of exchangeable eyepieces was not yet common in Messier's time.

Although some of Messier's reflecting telescopes had 7.5-to-8.0-inch aperture, they had little light gathering power as their mirrors were made of speculum metal (glass mirrors came in use only in the 1850s).

In his contribution to Sky & Telescope which is reprinted in Mallas' and Kreimer's Messier Album, Owen Gingerich points out that Messier's favorite instrument was a 32-feet FL, 7.5-inch aperture Gregorian reflector with mag. 104x, not listed above. Bailly has computed that the effective aperture of this instrument was equivalent to a 3.5-inch refractor. Even worse was the situation of the old Newtonian reflector he brought over from Delisle, which was an 8-inch but as effective as a 2.5-inch refractor only, so it was little used, although it seems this was the "original" instrument at Hotel de Cluny, Messier's observatory. Later he preferred to use several 3.5-inch (90 mm) achromatic refractors, which were usually about 3.5 feet long and magnifying 120 times. He selected to use these scopes because they were the best accessible instruments for him.

It remains to state that all of Messier's instruments could probably not compete with a modern 4-inch refractor or 6-inch Newton reflector. Therefore, even moderately equipped amateurs of current days can easily hunt down all the objects he observed and cataloged.