Well, Summer is finally over, for us who are in the Northern hemisphere the Autumn will start and our daylight periods will become shorter every day; the opposite will happen for our friends in the southern hemisphere, receiving the Spring and starting to have longer days. The day that marks this particular event is called the Equinox, and is when earth faces the Sun and receive its shining directly in the Equator, and Earth’s rotational axis is neither tilted away from nor towards the Sun; and therefore, its inclination doesn’t have an effect on the day, could be said that Earth faces the Sun sort-of “sideways”.
In this day, it is regarded that the south and Northern experience in this date a day where daylight and night duration is the same, although due to the effect of Earth’s atmosphere the days are yet a bit longer than the nights. Another fact is that equal daylight and night duration is effective only for those locations at the Equator, since it varies slightly for locations at different latitudes (locations that are more distant, north or south, from the equator).
Autumn equinox is considered the second of the year, the first being that of March 21-22. This date has significance to certain cultures, but overall this day doesn’t have a great impact or big drama.
But there are places where the equinox is far more notorious and absolutely with a bigger impact and significance; probably the most spectacular might be that of Saturn, the sixth planet in our solar system; but, why is that?
For start, Saturn with its peculiar set of rings has for most of their year, that last close to 29 ½ Terrestrain years, during their day they always have a shadow projected on their surface since sunlight is covered by their rings. Also, Saturn nights aren’t completely dark as most of the year the sunlight is partially reflected by its rings on the “night side” of the planet’s surface.
But this changes briefly during Saturn Equinox, when Saturn’s rings are parallel to the Sun’s rays and these don’t cast any shadow on the planet’s surface; and considering that Saturn rings are extremely thin, having from 10 meters to a maximum of 1 kilometer thick, the rings practically seem to disappear during Saturn equinox. Only the outer F ring that is slightly tilted compared to the rest gets some sunlight.
The Cassini space probe as part of its mission to Saturn had the opportunity to witness an equinox on this planet during August 2009, and aside from the mentioned absence of shadow on Saturn’s surface, it detected something magnificent. Being the Saturn rings so thin, any big structure within the rings cast a shadow on them, revealing magnificent features and small “moons”. In the case of the outer part of the B ring, a line of long shadows reveals a field of spikes formed in the edge reaching a height of up to two and a half kilometers.
Certainly, this is a more dramatic equinox than that of Earth. And considering the duration of Saturn’s year, a more elusive one.