The Google doodle above celebrates the first day of autumn, the autumn equinox.

On our annual journey around the Sun, the daily position of the Sun changes. Yes, it rises approximately in the east and sets approximately in the west throughout the year, but not in exactly the same place. If you observe it over time, you'll see the sunset direction becoming more northwesterly as autumn and winter progress, and then more southwesterly as spring turns into summer. Sunset is only due west, and sunrise is only due east near the time of the equinoxes.

What an equinox is
We get two equinoxes each year, one in spring and one in autumn. The autumnal equinox occurs around September 23 in the northern hemisphere, but it varies slightly. An equinox is also more specific than a particular calendar day. It's the time at which the Sun crosses the celestial equator. The autumn equinox occurs when the Sun crosses going south.

The celestial equator is the projection of Earth's equator into the sky, part of a celestial coordinate system that mirrors our earthly one. The ecliptic is the apparent path of the Sun through the sky — it's at an angle, because Earth is tilted on its axis. This is what causes the seasons. If your hemisphere is tilted towards the Sun, you have summer, and when it's tilted away you have winter. But for an equinox to occur, the direction of the tilt must be at a right angle to the Sun, so that it's not tilted towards the Sun at all.

The autumn equinox marks the start of autumn. The days will continue to shorten and the nights lengthen until the winter solstice.

What an equinox is not
"Equinox" means equal night in Latin. Despite what you may read, this doesn't mean that on the day of the equinox everywhere on Earth will have precisely twelve hours of daylight and twelve of darkness. It does mean that day and night are approximately equal in both hemispheres, but a number of local factors determine on which day a given place will have a day and night of twelve hours each.

At the time of the equinox neither hemisphere is tilted towards the Sun, so both north and south receive equal amounts of solar radiation. Despite what you might read on social media, none of this is related in any way to the Sun's gravitational force, and it does not affect Earth's magnetic field.

Ancient history
Mesopotamian civilization began over six thousand years ago in what is modern Iraq and parts of neighboring countries. When written history began, equinoctal celebrations were already taking place in this area.

Although there is nothing on paper, the importance of the equinox was marked in stone in Ireland over five thousand years ago. In Loughcrew (County Meath) there are a number of megalithic mounds, including one which is aligned to the equinox. It is 35 meters (115 feet) in diameter, but at the equinox the rising Sun shines through the mound and illuminates stone carvings not otherwise visible. [Image: Alan Betson]

Harvest home and the harvest moon
In Britain, one name for the traditional harvest festival is Harvest Home. It's still celebrated in some rural areas on the Sunday nearest the harvest moon, which is the full moon nearest the autumn equinox.

People picture the harvest moon as particularly large and reddish. Although the Moon doesn't really change its color or size, we do often see it this way. Around the time of the equinox the full Moon rises very soon after sunset. When it's low in the sky its light has traveled through a lot of atmosphere. The air acts like a filter, scattering away the bluer light and keeping the redder light. The Sun looks red at sunrise and sunset for the same reason.

There is also a common illusion that makes objects near the horizon seem bigger. If you hold out your hand at arm's length and use a fingernail to cover the Moon to get an idea of its size, you'll see that it's the same size higher in the sky, even though it looks smaller.

Autumn Moon Festival
Speaking of the Moon, one of the biggest autumn equinox celebrations is the Chinese Moon Festival. It's celebrated in China and wherever there are substantial Chinese populations. Its history goes back well over a thousand years and during that time it has collected around it a myriad of stories and customs. However it seems that for most people today it's a time of family togetherness and thanksgiving — and a chance to admire the full moon and perhaps eat moon cakes.

In China, contemplating the Moon is a venerable pastime which I can only applaud. Historic gardens and palaces have moon-viewing pavilions. Few people could afford to have one of these today, but in Golden Gate Park, San Francisco, there is a replica of a 13th century Ming dynasty garden and it does include a a traditional moon-viewing platform. I love to look at the Moon. This would be heaven.

Reference:
Sara Naumann, "Mid-Autumn or Mooncake Festival in China: Celebrating the Harvest Moon"
https://gochina.about.com/od/eventsfestivals/p/MoonFestival.htm