Blue Planet Project


 

We start this article continuing our trip along the Ecliptic, and finish by travelling across thousands of years.

The Constellations
I've included the Big Dipper, the best-known part of Ursa Major, the Big Bear, in this month's star map so you can stay oriented. This month's main constellation, Leo the Lion, is easily found in April - it's a bit south of the zenith, that point that is straight overhead. Look for the bright stars Regulus and Denebola, and the distinctive curve of the four brighter stars of the shoulder and head. Cancer, the Crab, is dim and hard to find unless you're someplace pretty dark. In suburban areas it looks like a starless patch between Gemini and Leo. There are two other really dim constellations on the map - Leo Minor (below Ursa major) and the Lynx.

Leo and Cancer are on the ecliptic, so keep an eye out for planets.

Leo represents the Nemean Lion, killed by Hercules as one of the twelve tasks set by jealous Hera. After that, Hercules wore the lion's skin as a cape. You meet Hercules in another article.

Cancer contains the beautiful star cluster M44, called the Beehive. If it's dark enough where you are, you can see this with just your eyes. Binoculars or a telescope at low power will show M44 as a beehive of stellar activity.

Time Traveler
Faint as it is, geography, time and gravity all meet in Cancer. The Tropic of Cancer is named for this little constellation. That imaginary line marks the farthest north the Sun can travel, where the Sun is straight overhead on the Summer Solstice, the longest day in our northern year. Thousands of years ago the Sun was in Cancer on the day of the Summer Solstice. Now Summer's first day finds the Sun in Gemini, on Castor's big toe, well west of Cancer. You can check it out on your astronomy software - go to noon on June 21, in this year, and find the Sun. Now switch the AD to BC. The sun will pop over to Cancer. Two thousand years earlier, the sun was in Leo when Summer began; 2000 years from now, it will mark the beginning of Summer in Taurus..

That our seasons drift through the constellations is interesting now, but when planting and harvesting and religious festivals were foretold by the stars, it began to cause considerable social disruption. Calendars had to be rewritten and festivals had to be rescheduled. The leap year had to be invented, too. All this because Earth spins like any toy top, with a little wobble induced by gravity. Over the years, that little wobble adds up to the westward drift of the seasons. The Sun moves through the ecliptic a constellation per month; the seasons drift more slowly, taking 2000 years for each constellation. We don't notice the seasonal drift now, since we have clocks, accurate calendars, and an understanding of astronomy.