This article is about an astronomical event. astronomical phenomena 2018 usno pdf March and 23 September. In other words, it is the point in which the center of the visible sun is directly over the equator.
69 seconds from the actual time of equinox. Sun’s center is not precisely over the equator at the moment of equinox. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox. Diagram of the Earth’s seasons as seen from the south. 45 BC, he set 25 March as the date of the spring equinox. 21 March, and by AD 1500 it had drifted backwards to 11 March.
This causes the equinox to oscillate by about 53 hours around its mean position. This in turn raised the possibility that it could fall on 22 March, and thus Easter Day might theoretically commence before the equinox. These are the historically universal and still most widely used terms for the equinoxes, but are potentially confusing because in the southern hemisphere the vernal equinox does not occur in spring and the autumnal equinox does not occur in autumn. It has become increasingly common for people to mistakenly refer to the September equinox in the southern hemisphere as the Vernal equinox. Although the terms have become very common in the 21st century, they were sometimes used at least as long ago as the mid-20th century. The northward equinox occurs in March when the sun crosses the equator from south to north, and the southward equinox occurs in September when the sun crosses the equator from north to south.
These terms can be used unambiguously for other planets. They are rarely seen, although were first proposed over 100 years ago. Day is usually defined as the period when sunlight reaches the ground in the absence of local obstacles. On the day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, so night and day are about the same length. Sunrise and sunset can be defined in several ways, but a widespread definition is the time that the top limb of the sun is level with the horizon.