Our autumn almanac 17.09.2010 – 23.09.2010

Autumn Equinox
Autumn begins on the equinox which takes place this year on 23 September at 03.09h GMT. So daylight lasts for 12 hours followed by 12 hours of night. Why do we have 24 hours in a day? More than 3000 years ago, in Egypt, the priests of Ra ruled that there would be 12 hours of the day and 12 hours of the night. Perhaps the ancient Egyptians had an obsession with the number 12 just like the Babylonians and the ancient Sumerians before them. The idea that counting began with the three sections of finger bone on each of four fingers, three times four making twelve, or the twelve constellations on the ecliptic, the path of the Sun across the heavens, so significant to Babylonian astrology, who knows?

The autumnal equinox is a good time to make a note of where east and west are on your horizon as you look out from where you live, because on this particular day the Sun rises due east and sets due west. Knowing where east and west, and therefore where north and south are, is very helpful in finding your way around the night sky when you are using a star map.

At this time of year the night sky seems to be full of the summer star patterns or constellations we could see a month ago. This is because the nights are falling much earlier as autumn begins and this means we still see the summer constellations in the evening sky as they move towards the west.

This time of year has more to do with harvest than summer. As the ancient Greek poet Hesiod has it: “when Orion and Sirius move into the middle of the sky, and the star Arcturus sees the rosy fingered dawn, then pluck the clustered grapes and bring your harvest home”.

Harvest Moon
This year we see the Harvest Moon rise at 9.20 on the same day as the equinox, a rather special event as the equinox will not fall at the same time as the harvest moon again until 2029.

When the Full Moon rises its brilliance brightens the fields of harvest all night long. In the days before the mechanization of agriculture the harvest was all collected by hand. Everyone was needed to bring the harvest home as quickly as possible, and the Harvest Moon allowed people to work in the field well into the evening and late into the night.

This special moon also marks Wednesday’s mid-autumn Festival, celebrated by Chinese people worldwide at this time of year. It’s a festival marked with round moon cakes and lanterns to represent the harvest moon. The shape of the full moon is significant, as the word “round” implies family reunion in Chinese.

The Craters of the Moon
This week NASA reported on their latest survey of the craters of the Moon using advanced measuring techniques using lasers.

The violent history of our nearest celestial neighbour has been laid bare by the most detailed map of moon craters ever produced.

Scientists used instruments aboard Nasa’s Lunar Reconnaissance Orbiter to scan the surface of the moon for impact craters measuring at least 20km wide.

Pictures sent back by the spacecraft revealed 5,185 large craters caused by lumps of space rock thumping into the lunar surface over the past few billion years. Some regions of the moon are so pocked with craters they have reached what planetary scientists call “saturation equilibrium”, where each additional crater wipes out an older one, so the number of craters remains the same.

The moon is thought to have formed 4.5 billion years ago, when a heavenly body the size of Mars struck Earth and dislodged an enormous cloud of debris that ultimately condensed into our planet’s natural satellite. By analysing the craters and their positions, the researchers determined that the oldest regions of the lunar surface were the southern areas that face Earth and the northern region of the far side of the moon. Some parts of the moon are younger than others because ancient volcanic eruptions spewed out material that covered vast areas of land and erased the craters that were there before.

The map confirms previous lunar surveys that found older parts of the moon’s surface have a greater number of craters than younger areas. This suggests the moon was pummelled with larger space rocks in its early life than it was later on.

One possible explanation is that fewer huge chunks of rock were flung out of the asteroid belt and onto a collision course with the moon once Jupiter and Saturn – the planets with the most mass and so the greatest gravitational pulls – had settled into their orbits.


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