Fall equinox marks the beginning of autumn, but not equal day and night

At 3:05 Monday morning, the Sun moved directly over the equator on the other side of the world, over a point 130 miles north of Papua New Guinea, marking the astronomical beginning of autumn in the northern hemisphere.

That subsolar point has been moving southward since it paused for a moment over the Tropic of Cancer at the June Solstice which marked the beginning of summer. It will now continue southward until the December Solstice when it will pause over the Tropic of Capricorn before moving northward in this yearly cycle.

The word equinox comes from the Latin word equinoxium, meaning "equal day and night." Looking at the Earth from space today, the northern and southern hemisphere receive about the same amount of daylight. But that doesn’t mean we’ll get exactly 12 hours of sunlight here in Raleigh on the day of the equinox.

The day where that daylight comes closest to 12 hours, also known as the equilux, arrives in central North Carolina on Thursday. The Sun will be above the horizon a few seconds more than 12 hours.

Equilux can occur as early as late August just north of the equator in Columbia and Venezuela and as late as mid-October in northern Peru and Brazil. Areas of the globe that lie on the equator will never see the Sun above the horizon for exactly 12 hours because of atmospheric refraction.

We actually see the sunrise before the upper limb of the Sun is physically above the horizon. The atmosphere bends the light about half a degree, moving the time we see the sunrise earlier and allowing us to see the Sun after it has physically set.

You may have also heard that the Sun rises due east on the day of the equinox. Not exactly. The equinox occurs at the same time globally. The closer that time is to sunrise at your location, the closer that sunrise will be to due east.

Here in central North Carolina the sun will rise a fraction of a degree away from due east on Tuesday, Sept. 24.