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May 2005 Feature: the Milky Way's Disappearing Act


At our mid northern latitudes during the month of May, the great band of stars known as the Milky Way simply vanishes from the evening sky. On May evenings, the plane of the Milky Way coincides with the horizon in all directions. The northern terminus of the Milky Way -- the W-shaped constellation Cassiopeia the Queen -- sits on your northern horizon. Meanwhile, the constellations Crux (the Southern Cross) and Musca the Fly -- the southern terminus of the Milky Way -- lurk southward, exactly opposite of Cassiopeia.

The southernmost regions of the Milky Way climb highest into the evening night sky at this time of year. Folks residing in the subtropics and tropics of the Northern Hemisphere can view Crux, or the Southern Cross, at evening right now. When Cassiopeia sinks beneath your northern horizon, the Southern Cross rises above your southern horizon. If you live in Hawaii, or the southern tips of Texas and Florida, you have a good chance of seeing the Southern Cross. Click here for more on Cassiopeia and the Southern Cross.

On May evenings, the equator of the Milky Way circles the rim of the horizon, with the North Galactic Pole standing high overhead in the constellation Coma Berenices, or Berenice's Hair. In this direction, where the glare and the dust of the Milky Way are minimal, the sky beckons you to look at the deep-sky objects beyond the Milky Way. As seen from the North Galactic Pole, the Sun and the solar system revolve clockwise around the center or nucleus of the Milky Way Galaxy.

When the Milky Way rims the horizon on a May Day evening, that means the year is about halfway between the March equinox and the June solstice. It's easy to imagine that the hoarfrost of winter is giving way to the fireflies of summer.

Six months from now, the equator of the Milky Way will circle the horizon at mid southern latitudes. On November evenings -- when the constellation Cassiopeia sits highest in the sky -- the beautiful Ethiopian Queen can be seen from the tropical and subtropical regions of the Southern Hemisphere.

Mars in Aquarius

Mars passed from the constellation Capricorn and into the constellation Aquarius on April 27. Mars will stay in front of the stars of Aquarius until June 7, when it will enter into the constellation Pisces. At mid month, Mars and Uranus will rendezvous, with the two planets standing together within a single binocular field. Throughout May, Mars rises after midnight and adorns the southeast sky before dawn.

On and near May 24, Mars passes near the three Psi stars of Aquarius, which can be located on this Aquarius sky chart. (It's the closely-knit group of three stars at the upper left of the drawn-in loop of stars.) In 1672, astronomers were able to figure out the distance to Mars by noting the slightly different position of Mars relative to these Psi stars from two different locations on Earth. The last time that Mars passed by these Psi stars was in November of 2003, which inspired an article on this historic accomplishment.

Incidentally, Christiaan Huygens (1629-1695) proposed to the Royal Society in 1661 that one swing of the pendulum should be taken as the universal standard for one second of time. But when Jean Richer (1630-1696) traveled to the island of Cayenne in 1671, getting ready to make his observations of Mars the following year, he found that he had to shorten the pendulum by one tenth of an inch to have it coincide with one second of time.*

Occultation of Antares

For North America this month, the Moon occults (passes in front of) the star Antares of the constellation Scorpius. For the West Coast, it happens late at night on May 23; and for the East Coast, the occultation happens in the wee hours of the morning on May 24. Specific times and a map of this occultation are given at this International Occultation Timing Association page. Remember that the listed times are in UT (Universal Time). To convert to your time zone, subtract 4 hours for EDT (Eastern Daylight Time), 5 hours for CDT, 6 hours for MDT and 7 hours for PDT. For example, at New York City, NY, Antares disappears behind the Moon at 8:21 UT and reappears at 9:21 UT. Since NYC resides in the EDT zone, subtract 4 hours to find that Antares disappears at 4:21 a.m. and reappears at 5:21 a.m.


* page 169 of Cambridge Illustrated History of Astronomy, edited by Michael Hoskin

copyright 2005 by Bruce McClure

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April 2005 Feature * June 2005 Feature