Zero Hour

This time we take a fishing trip as we continue our tour of the faint and obscure constellations.

Pisces is well known, as it’s one of the Zodiac constellations. These constellations had (and unfortunately are still given) astrological significance; they lie on the imaginary line of the Ecliptic. To astronomers ancient and modern, the Ecliptic is the path followed by the planets and the Sun and, more loosely, our Moon. Since Copernicus, we’ve known it’s the plane of Earth’s orbit projected out into space. Not surprisingly, it cuts the celestial equator at an angle of 23.5 degrees – the same as Earth’s tilt. The ecliptic and the celestial equator currently meet in Pisces at a point known, oddly enough, as “the First Point in Aries.”

The dashed line marked 0h on the diagram is the celestial Prime Meridian – the Zero-hour line. This line joins the poles with the First Point in Aries. It does the same job as Earth’s zero meridian, the one that goes through Greenwich, England. The “longitudes” of objects in the sky are measures East and West of this line as hours and minutes, just as earthly longitudes are measured east and west of Earth’s prime meridian. The nearer point where the ecliptic crosses Pisces marks One Hour (1h), while the sharp angle in the constellation lies on 2h.

When the Sun, in its annual wander along the ecliptic, crosses the equator coming north, our Spring begins. The day and night are equal length, and we say we are at the Spring (or Vernal) Equinox (equi-equal nox-night). This happens when the Sun passes through the First Point in Aries. You can probably guess now why it’s called that – it has to do with the constellations’ slow drift across the sky caused by earth’s precession. Thousands of years ago, the First Point in Aries was, in fact, in Aries.

To see this in your astronomy software, turn on the celestial grid lines, the ecliptic and the constellation lines. If you can, turn on a label for the vernal equinox, too. Set the date and time for noon on March 21; the Sun should be on the Vernal Equinox. Now switch the AD to BC and watch the equinox and the Sun jump back into Aries. You might have to adjust the date by a couple of weeks to get everything aligned, though.

Deep Sky Effect
As I pointed out above, the zero meridian is defined by the line through the poles and the first point in Aries, and that point is moving. One side effect of this is seen in the arrangement of the NGC deep-space objects. They were numbered rationally, starting near the north celestial pole at the zero hour meridian. From there the numbers increase toward the south pole, then move a bit west and so on. By the time the whole sky has been traversed, back around to the 23h59m meridian, you’d be at the last-numbered NGC object. However, as you’ll see in any good star chart, it’s not quite like that; the precession of the equinoxes (i.e. earth’s wobble) has moved the zero meridian enough since the creation of the New General Catalogue (for that is what NGC stands for) that the highest-numbered NGCs are now to the east of the line. The precession is also why sky atlases have to be updated a couple of times a century.

Related Book Review
For a wonderful account of the competition to find a way to measure longitude, read Dava Sobel’s “Longitude”. It’s a tale of a huge prize, scientific rivalry, bias and greed. Even Galileo got in on it, thinking that the moons of Jupiter, which he had just discovered, offered a way to measure longitude. They didn’t, but years of measurements of the moons’ orbits gave the first direct evidence that the speed of light is finite. It’s a fascinating book, and is probably in your local library.

Sky School