An asteroid measuring 250 meters in diameter passed within 280,000 miles of Earth last May 19. The stadium-sized rock was about the same distance from Earth as the moon, the closest approach ever observed for such a large asteroid. This may seem like a comfortable distance but, taking into consideration that the average velocity of the Earth in orbit is 18.5 miles per second, the time difference to a catastrophic collision was only 4.2 hours.

The asteroid was large enough to destroy a large city if it had impacted Earth and have shot enough debris into the atmosphere to reduce global temperatures by several degrees. As a TV character used to be fond of saying, “Missed us by that much!”

FOCUS ON THE PLANETS

MERCURY may be found low on the eastern horizon about an hour before sunrise as the month begins. It will be most prominent on Oct. 3 before beginning a month-long descent into the sunrise.

VENUS continues to dominate the eastern horizon at dawn where it has a close conjunction with Regulus on the morning of Oct. 4.

MARS is above, but much dimmer than, Venus in the east before dawn. The red planet is slowly moving towards Regulus as Venus drifts away.

JUPITER is in the southwest at nightfall surrounded by the stars of Sagittarius. Lower and dimmer than it was in the summer months, Jupiter is still the brightest object, except for the moon, in the night sky.

SATURN is well up on the southeastern horizon as darkness falls. By midnight, the ringed planet is due south where its slight tilt offers a unique opportunity to view both the planet’s surface and ring system.

URANUS and NEPTUNE are in the south situated between Jupiter and Capricornus. A good telescope should reveal the greenish disk of Uranus while bluish Neptune hovers nearby.

PLUTO is, as usual, a lost cause to all but the near-professional stargazer.

FOCUS ON A CONSTELLATION

The sun enters Libra the Scales on Oct. 30 making it this month’s stop on our yearlong journey through the constellations of the zodiac. Libra is a small constellation whose dim stars take the shape of a lopsided rectangle. Unprepossessing in appearance, it has a rich mythology surrounding it.

Some 3,000 years ago, the autumn equinox occurred in Libra giving it a special significance. The ancient Babylonians introduced the idea of Libra as being a scale and the Egyptians used it to symbolize the weighing of grain at harvest time. By 200 B.C., the stars were widely described as being the tips of Scorpio’s claws but legends of a scale were not forgotten. Libra officially became the Scales in 46 B.C. when it became part of the Julian calendar. Statues of the Emperor Augustus holding the scales aloft as a symbol of justice date from that time and the “scales of justice” have been with us ever since.

OCTOBER EVENTS

1. Sunrise, 6:33 a.m.; sunset, 6:17 p.m.

3. Look for Venus very near Regulus, the heart of Leo the Lion in the eastern pre-dawn sky.

4. Moon in last quarter, 8:05 a.m.

7. Venus, Regulus, and Mars form a vertical line up from the eastern horizon at dawn.

12. New moon, 10:13 a.m.

18. Look to the southwest for the moon to the upper left of Jupiter. If you use binoculars to carefully scan the sky above and to the right of the moon, you may be rewarded with a glimpse of comet Hale-Bopp.

23. The sun enters the astrological sign of Scorpio but astronomically is still in Virgo.

26. Full moon, 10:10 a.m. As the first full moon following the harvest moon on Sept. 27, this month’s is the hunter’s moon.

27. Don’t forget to set your clocks back one hour as we go from daylight-saving time to standard Time.

29. Mars, Regulus, Aldebaran, and the moon form a great arc in the predawn sky extending from the eastern horizon, through the zenith, to the western horizon.

30. The sun enters the constellation of Libra on the ecliptic.

31. Halloween, the eve of All Saint’s Day, a cross-quarter day marking the midpoint between the autumn equinox and the winter solstice. Sunrise, 6:12 a.m.; sunset, 4:26 p.m.

Clair Wood’s astronomy column is a monthly NEWS feature.