Watch the Full Moon rise on Good Friday evening.
See the end note about enlarging illustrations.
In the previous night the Moon passed 7° north of Spica (Virgo’s “lucida” or brightest star) at 3h by Universal Time (5 or more hours earlier by clocks in North America). Then in the daytime it was at the exactly full phase at 11 UT, about 15 hours before the time of our picture.
The Sun has to be diametrically opposite to the Full Moon. See the “anti-Sun” marked at the time of the picture.
It happens this year that the Sun, at almost exactly the same instant as Full Moon, is entering the constellation Aries.
But, according to the ancient and astrological system of dividing the zodiac into 30°-wide “signs,” the Sun is entering the sign Taurus (on April 20 at 9 UT). That system was codified about two thousand years ago; since then, precession – the slow twisting of the spinning Earth’s axis – has twisted the whole sky around, or rather our map of it. This Full Moon gives you a clear opportunity to see the difference between the two systems – “signs” and the physical constellations.
The point in the sky where the ecliptic and the celestial equator cross is marked “equinox point” because it is where the Sun is at the September equinox. Diametrically opposite to it is the other equinox point, where the Sun is at the March equinox. But traditionally that point is called “The First Point of Aries,” because it was in Aries two thousand years ago.
That curiosity of terminology is quite well known; less often mentioned is that the point in our picture, the September equinox point, was called the “First Point of Libra.” Libra is the constellation opposite to Aries, and is where the Sun was at the autumn equinoxes of long ago. It is below the horizon of our picture – I’ve brought it into visibility from underground.
Contemplate the distance from the Moon to the First Point of Libra. It’s an imaginary point, you can’t see it: use this 15° measure at arm’s length –
– which is the same as the arrow showing the angle the sky turns in an hour. Make about two and a half of those hand measures up from the Full Moon. That’s the distance by which the point has precessed westward in two thousand years. Back at the time if the events that Good Friday and Easter commemorate, the crossroad of ecliptic and celestial equator was where the Full Moon is now.
Yes, our Moon, marking the ancient First Point of Libra, is still in Virgo. That’s because, whereas the “signs” are 30° wide, the constellations are irregular. Virgo is by far the largest of the zodiacal twelve, and steals some of the ecliptic from Libra.
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DIAGRAMS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “View image”, then enlarge. Or choose “Copy image”, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). I am grateful to know of what methods work for you.
Very helpful and interesting. If the equinoxes are precessing, why are there 3000 year old temples in Egypt where the sun shines straight down a long narrow tunnel on the date of each equinox? Still, after 3000 years.
The Sun on the equinox or solstice date of a different era is in a different position against the starry background, but I think it is in essentially the same geometrical position in relation to Earth and locations and objects on it. Not precisely, because the Earth’s inclination changes very slightly.