It is so astronomically special because the time of the Moonrise to the subsequent Moonrise is longer in the Southern Hemisphere. They call it the Harvest Moon in the Northern Hemisphere, but we call it the September full moon. The September Moon is astronomically special. It is all to do with the tilt of the Earth. So if you want to be up at 5:21 am on Thursday, September 23rd, because it’s at this instant that the Earths rotational axis is neither tilted away from nor towards the Sun. So even though we often celebrate the Equinox as an all-day event, it is only a minute. If we are in the Northern Hemisphere are around the same thing, it will be on September 26 th. In Sydney or Coonabarabran or in that general vicinity of about 35 degrees south latitude, the Equilux will eventually occur on September 19 th. This day depends on the locations latitude and can never occur several days or even weeks before or after an Equinox. There are days when sunlight and nighttime hours are very, very close to 12 hours. Another thing of interest is that even if day and night aren’t exactly equal on the day of the Equinoxes. When the calculations are done, the standard day at sea level of 1013.25 kPa and 15 degrees Celsius is used. Now the extent of the refraction depends on atmospheric pressure and temperature. This causes every day on Earth, including the days of the Equinox, to be at least 6 minutes longer than it would have been without this refraction. The same thing can happen at Sunset, where you can see the Sun for several minutes after it has dipped below the horizon. This refraction or the bending of the light causes the Sun’s upper edge to be visible from the Earth several minutes before the edge reaches the horizon. In that case, this is the time it takes the Sun to set fully, which can take several minutes, making the day just a little longer than the night on Equinoxes.Īnother reason the day is a little bit longer on the Equinox is that the Earth’s Atmosphere refracts the Sunlight. Suppose Sunrise and Sunset were defined as the moment the geometric centre of the Sun passes the horizon. So, the reason that most locations on Earth do not enjoy precisely 12 hours of the day and 12 hours of nighttime on the Equinox is how we define Sunrise and Sunset. Whereas if we lived on the equator, the days are always a little longer than 12 hours all year round. In Sydney, on the day of the Equinox, our day will be 12 hours and 6 minutes long mind you, if we are at the South Pole, our day would be up for 24hours, a perfect opportunity to see the midnight Sun. Our Northern neighbours commemorate the start of Spring on the date of the Equinox. Thanks to the Earth’s atmosphere and our definition of Sunrise and Sunset, Now in Australia, we celebrate the beginning of Spring on September 1st. Most Earth locations tend to enjoy more daylight hours than nighttime hours on those two nights of the year. The night and day lengths are only nearly equal on the September and the March Equinoxes. However, even though this is widely accepted, it’s not entirely accurate. Equinox is derived from the Latin ‘aequi’ and ‘nox’ meaning “Equal Night”. It means the nights are the same length (about 12 hours) all over the world. Still, on both Equinoxes, the Earth’s axis’s tilt is perpendicular to the Sun’s Rays. The Southern Hemisphere or the Northern Hemisphere tilts a little more towards the Sun. The Earth’s axis is tilted at an angle of 23.4 degrees to the ecliptic on any other day of the year. The celestial equator (ecliptic) is an imaginary line in the sky by the Earths Equator from North to South. The time for us in Eastern Australia will be at 5:21 am AEST. So the September Equinox occurs when the Sun crosses the celestial equator. Earth rotates on its axis as it orbits the Sun, but the axis always points in the same direction. Figure 1: Earth has seasons because its axis is tilted.
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