It is common for major league baseball players to be sent to the minors, but Pluto’s recent demotion from planetary status is only the second time such an event has occurred. When Ceres was discovered in 1801 between the orbits of Mars and Jupiter, it was first classified as a planet because its orbit fit exactly with where astronomers predicted a planet should be located.

Within 50 years it was realized that countless objects occupied what it now known as the asteroid belt and Ceres was downgraded to the largest and most massive of them. Pluto lost planetary status because it failed the third of three recently passed rules for planets. These are the body: (1) must orbit the sun, (2) must be massive enough so self-gravity makes it spherical, and (3) must have cleared its orbital neighborhood of other bodies.

Don’t throw out your old charts of the planets just yet, however, for the fight to reinstate Pluto is likely just getting under way. Dr. Alex Stern, head of NASA’s New Horizon’s mission to Pluto, scoffs at the decision. He noted that only 424 astronomers, less than 5 percent of the world’s total, voted. As for rule three, Stern points out that Jupiter has thousands of Trojan asteroids in its orbital neighborhood plainly failing the third test.

Focus on the planets

Mercury is so low on the western horizon this month that it is all but invisible to midnorthern latitude viewers. The best bet to see Mercury is on Sept. 24 when, about half an hour after sunset, it peeks above the horizon to the lower right of the thin crescent moon.

Venus shines brightly on the pre-dawn eastern horizon as the month opens. On Sept. 1, Saturn lies to the upper right of Venus. The time between Venus rising and sunrise narrows each day until Venus drops from sight in late September.

Mars grows steadily fainter and, as it sets almost at the same time as the sun, it is lost to view in midnorthern latitudes this month.

Jupiter opens the month low in the southwest shortly after sunset. Jupiter sinks steadily during the month and surface details such as the Great Red Spot are obscured by the thicker Earth atmosphere, but the always fascinating dance of its four major moons will still be visible.

Saturn returns to the pre-dawn eastern horizon in September. Rising higher and earlier each day, Saturn appears four hours before sunrise at month’s end affording an excellent opportunity to view its ring system.

Uranus is high in the south, amid the stars of Aquarius, around midnight. A medium-size telescope will reveal the blue-green disk of Uranus.

Neptune is located in the constellation of Capricorn where its blue-gray disk is visible by telescope. Both Uranus and Neptune may be found with the aid of a finder’s chart such as in the May issue of Sky & Telescope.

Pluto gets a final farewell as a planet and is located high in the southwest during the early nighttime hours, but an 8-inch minimum telescope and the finder’s chart will be needed to spot it.

September events

1 Sunrise, 5:57 a.m.; sunset, 7:13 p.m.

7 Full moon, 2:43 p.m. The full moon of September is called the Fruit Moon.

8 The moon is at perigee, or closest approach to Earth, today. This fact, coupled with the recent full moon, may lead to abnormally high tides.

14 Moon in last quarter, 7:17 a.m.

17 The sun enters Virgo on the ecliptic.

18 Saturn appears directly below the thin crescent moon tonight. Venus is well down to the lower left, just above the eastern horizon, with Regulus sandwiched in between.

22 New moon, 7:45 a.m. The moon is at apogee, or greatest distance from Earth, today.

23 Fall or autumnal equinox, 12:03 a.m. The sun crosses the equator into the southern hemisphere. The sun rises due east and sets due west everywhere on Earth, except for the poles, today. The sun enters the astrological sign of Libra but astronomically has only just entered Virgo.

24 Look to the southwestern horizon about a half-hour after sunset where Mercury may be spotted to the right, and a bit lower, than the thin crescent moon. Don’t mistake Spica, situated between the Moon and Mercury, for the elusive innermost planet.

25 Find the moon on the southwest horizon about an hour after sunset. The shining “star” to the moon’s upper left is Jupiter.

27 The bright star to the upper left of the moon as dusk deepens is Antares.

30 Moon in first quarter, 7:03 a.m. Sunrise, 6:31 a.m.; sunset, 6:18 p.m.

Send astronomical queries to Clair Wood at cgmewood@aol.com or care of the Bangor Daily News, Style Desk, P.O. Box 1329, Bangor, Maine 04402.