Wednesday marks the beginning of autumn, and the official beginning of that time of year when the air turns nippy, you have to scrape frost off your car in the morning, the leaves change color and drop from the trees, and the moon seems bigger and brighter than usual.

Wednesday is considered the beginning of autumn in the northern hemisphere because it’s the day of the autumnal equinox, the date when day and night are equal in length. Normally that’s not true. The Earth’s axis is tilted. Because the Earth revolves around the sun once a year, that means that during parts of the year we’re tilted toward the sun, and during other parts of the year we’re tilted away from it. The autumn equinox is one of two moments during the year when we’re neither tilted toward nor away from the sun; the other is the spring equinox. (And in the southern hemisphere, of course, Wednesday is the beginning of spring, instead of autumn.) For the six months after the spring equinox and March and the autumn equinox, our days are longer than our nights; from now until the next spring equinox, our nights will be longer than our days.

The lengthening nights at this time of year account for many of the other phenomena associated with autumn, beginning with the fact that it’s starting to get colder. There are two reasons for this. One is that, once the sun goes down, the Earth radiates heat into the blackness of space–the night sky. That’s why it cools down every night. Since the nights are now getting longer, there’s more time every night for the Earth to radiate away the heat it picked up from the sun during the day, and so the nighttime temperatures are falling off rapidly.

The second reason for cooler days is that, since we’re no longer tilted toward the Sun, the sun’s rays are hitting us at much more of an angle than they did at the height of summer. That means they have a thicker layer of atmosphere to go through before they get to us, and passing through the atmosphere saps them of some of their energy. At the height of winter, the sun blazing away, low to the southern horizon, seems to produce no heat at all for that reason.

Longer nights and shorter, cooler days also trigger the archtypical autumn phenomenon: leaves changing color and falling from the trees.

Leaves are normally green because they contain chlorophyll. However, they also contain other pigments, especially yellows, oranges and reds, which are masked most of the time by chlorophyll. What happens in autumn is that trees shut down food production for the winter. Chlorophyll fades from the leaves, and the other pigments come to the fore.

Eventually the leaves fall off deciduous trees altogether. By losing their leaves, the trees reduce water loss during the cold, dry days of winter. The same sort of process accounts for grass and other perennial plants turning brown. They, too, are shutting down food production and preparing to hunker down in their roots, which, protected by the soil around them, maintain the necessary spark of life to resurrect them in the spring.

To set off the process of dropping leaves, trees produce a hormone called abscisic acid. This causes leaves to form an “abscission layer” at the base of their stems. The cells of this layer don’t hold together very tightly. As they pull apart they weaken the stem, and eventually the wind blows the leaf off the tree. The falling of leaves hastens after a hard frost because freezing damages plant cells, causing the stem to weaken further. (Freezing causes damage to plants because plant cells are mostly made out of water, which expands when frozen. This causes the cells to rupture and die.)

Frost forms when water vapor in the air freezes onto cold objects. Warmer air can hold more water than cooler air, so as air cools, it eventually reaches the point where it can’t hold the moisture that’s already in it. That’s the dew point, the temperature at which dew — or, if it’s below freezing, frost — forms.

Cars, tree branches, grass blades and other things not in direct contact with the ground frost first, because whereas the ground stores some of the day’s heat, most objects quickly radiate it into the sky. (Eventually, so does the ground, as noted earlier.) Heat radiates more quickly into a clear sky than a cloudy one, which is why clear nights are the coldest.

As noted, frost kills plants: still, even though we know it means the end of flowers and the dropping of leaves, frost can be a lovely thing, especially glittering in the light of a harvest moon.

The harvest moon is the full moon occurring just before the autumnal equinox. Around this time the moon rises directly opposite to the sun, close to the exact eastern point of the horizon, shortly after sunset, which means you get strong moonlight almost all night long. Since this is a time when farmers are working late in their fields, the moonlight is a big help: hence the name “harvest moon.” The next full moon, which is similar, is called the hunter’s moon.

Autumn has always been hunting season because it’s a season for animal migrations–especially of birds.

Birds migrate to escape the cold and the dark, just like many humans do. The champion migrator is the Arctic tern, which spends summers in the Arctic, then flies 6,000 kilometres to winter in the Antarctic–where, of course, it’s summer again. That’s quite a feat. It’s still not clear exactly how birds pull it off, but it’s thought they use one of, or a combination of, piloting and orienteering.

Piloting means flying from one easily remembered landmark to the next. Since this kind of information can’t be passed on genetically, birds that use piloting have to learn the route from older birds, usually their parents.

Some birds fly long distances over open water: that requires orienteering, taking cues from environmental elements such as prevailing winds, characteristic cloud formations, even the stars. Pigeons orient themselves to the sun and even to the Earth’s magnetic field.

It doesn’t take perfect understanding of the mechanism of migration to appreciate its effects: the long skeins of geese flying in V-formation across the sky, calling to each other; the ponds and lakes crowded with thousands of birds; and that pleasantly melancholy feeling all this provokes in the human heart, as the year dies into winter once again.

Autumn, you see, can make even the most scientific soul wax poetic.

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