(from our friends at OMSI)
Saturday, March 20 is the vernal equinox for Pacific Time Zone at 10:34 a.m. PDT, the day on which both the north and south pole of the earth are equal distances towards the sun (92.6 million miles). At that instant the sun stands directly over the Earths equator. The first day of spring, called ‘the vernal equinox’, vernal meaning ‘green’, and equinox meaning ‘equal night’, which simply means that on the equinox the hours of daylight are equal to the hours of night.
- As seen from Portland on March 20, the noon sun (1:18 pm) will reach its mid-point in the sky near 45 degrees from the southern horizon.
- On the first day of spring, the Sun rises exactly due east and sets exactly due west. And each successive day thereafter it rises and sets just a little bit farther to the north until the summer solstice on June 20, the first day of summer, when the Sun reaches its northernmost point along the horizon and actually seems to ‘stand still’ and rise and set in the same place for a few days. In fact the word ‘solstice’ means ‘sun stands still’.
- As the season changes from summer to winter and vice versa, the Sun and Moon are in perfect balance, as if they were on opposite ends of a celestial see-saw:
- Summer: The Sun rises in the NE and sets in the NW; the Moon rises in the SE and sets in the SW. At transit, the Sun’s altitude is high; the Moon’s altitude is low. The Sun is visible ~15 hr.; the Moon is visible ~9 hr.
- Winter: Sun rises in the SE and sets in the SW; Moon rises in the NE and sets in the NW. At transit, Sun’s altitude is low; Moon’s altitude is high. Sun visible ~9 hr.; Moon visible ~15 hr. Note: The preceding statements are true only during (or near) Full Moon. During (or near) New Moon, the Moon appears to closely follow the path of the Sun across the sky.
- The lengths of day and night are then equal over almost all Earth, except at the poles. At the North Pole and South Pole, Earth’s atmosphere bends the Sun’s rays enough to make the Sun visible throughout the day and night, even during the 12 hours the Sun is below the horizon.
- From March 21 until September 24, the days are longer than the nights for the northern hemisphere. The 12 hours day and night actually occurs few days before the vernal equinox. This is due to the earth’s atmosphere causes the light from the sun to be refracted when the sun is near the horizon.
- In A.D. 150, the annual path of the Sun against background stars was such that on the vernal equinox, the Sun “entered” the constellations Aries. This is how the first day of spring became endowed with the name “first point of Aries.” In 1930, the International Astronomical Union restructured constellation boundaries. And because of the ongoing precession (wobble) of the Earth on its axis the “first point of Aries” has shifted in the calendar and occurs one month earlier than the vernal equinox. On the first day of spring, the sun will be in the constellation Pisces, the “Fish.” In about 600 years, the “first point of Aries” will reach the constellation Aquarius and enter the “age of Aquarius.”