Monday 21 September 2015

September Equinox 2015.

The September Equinox will fall on Friday 23 September 2015, when the day and night will be of equal length in both of the Earth's hemispheres. The Earth's seasons are driven by the tilt of the planet, but the planet does not, as is generally assumed, tilt back and forth over the course of a year. Rather it remains at a constant angle of 23½° to the plane of its orbit throughout the year (this does alter on a longer cycle, but not one that matters on human timescales), but this tilt is disconnected from its orbit about the Sun, with the effect that the Northern Hemisphere is presented towards the Sun from one side of the orbit (creating the northern summer and the southern winter), and the southern hemisphere from the other (creating the southern summer and the northern winter).

Simplified diagram showing the tilt of the Earth throughout the year. Not to scale. NASA.

At two points in the year the Earth presents the two hemispheres equally to the Sun (though it is still tilted at 23½° to the plane of its orbit), creating the equinoxes, which fall in March and September each year. The September Equinox, marks the end of summer in the northern hemisphere (where it is the Autumn or Fall Equinox) and the beginning of summer in the southern hemisphere (where it is the Spring Equinox).

See also...

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The Earth will reach its aphelion, the furthest point in its orbit from the Sun, a distance of 152 093 481 km, at 7.41 pm GMT on Monday 6 July 2015. The Earth's orbit is slightly eccentric and slightly variable, leading to the distance between the Earth and the Sun varying...

The June (or Northern) Solstice falls on Sunday 21 June in 2015, the day on which the Sun rises highest in the sky and the longest day of the year in the Northern Hemisphere (where it is the Summer Solstice) and...

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