2020, Early February: Daylight and Darkness Equal


During early February, the sunrise point and the sunset point move rapidly northward. The sun’s daily arc across the sky is higher and longer.  Simultaneously, nighttime shortens.  Daylight and nighttime are balanced at the equinox. The start of Spring occurs on March 19, 2020, when their lengths are equal.

Recently, though I have been considering another equal-time event.  That is, the length of daylight and the length of darkness – the time after the end of evening twilight and the beginning of the next morning’s twilight.

Twilight occurs when the atmosphere is illuminated by the sun when our central star is below the horizon.  Immediately after sunset, the sky is bright, especially in the western sky.  It continues to darken until the sky’s illumination ends. This takes about 90 minutes, longer during the summer months as seen from the mid-latitudes. This reverses in the morning until sunrise.

So a 24-hour cycle has three phases, daylight, twilight, and darkness. Traditionally, twilight and darkness together make night. 

At the mid-latitudes, the length of daylight and darkness are equal at about 10.5 hours.  At my latitude, this occurs on February 11, 2020, with 10 hours, 26 minutes of daylight and the same length for darkness.

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