Celestial Directions

We're familiar with three dimensional rectangular coordinates of width, depth and height or symbolically X, Y and Z. Rectangular systems work fine locally but are very unwieldy across large distances on spheres like the Earth. Describing the course of a plane from New York to Budapest in rectangular terms would be bewildering. On globes it is convenient to use spherical coordinates. North/South distances are measured in degrees of latitude. East/West distances measured in degrees of longitude. Altitudes can be measured from either the center or the surface of the Earth as desired.

When measuring things in the sky, astronomers use three separate spherical systems. The alti-azimuth system is based on the local horizon with an observer such as yourself as the center. The equatorial system is centered in the Earth. The ecliptic system is centered in the Sun. The choice of which system we use depends on our purpose.

If I wanted to tell you where to find Saturn in the early evening this Fall, I would say it was low in the southeast. I might be more precise by specifying so many degrees above the horizon at a bearing of so many degrees from due North at some specific date and time. In either case, we would be using an alti-azimuth system when we talked about where to find Saturn.

If Mars is in Leo, we must know when Leo is in the sky to know if Mars is visible. The Earth turns on its North/South axis like a giant one day clock with the Equator as its face. To calculate where Leo is, a celestial equatorial system is most convenient. The celestial equatorial coordinates are just projections of the familiar Earth based latitude and longitude lines. Imagine the Earth is transparent with a black paint on the latitude and longitude lines. Imagine further that the celestial sphere is a reflective globe. If a bright light is placed at the dead center of the Earth, then the shadows of the black lines on the celestial sphere would mark the corresponding celestial latitude and longitudes.

As Copernicus learned centuries ago, making calculations about planetary positions using an Earth centered system is extremely difficult. Most of the difficulties vanish as soon as we use a Sun centered system. The one we use is the ecliptic system named after the plane of the Earth's orbit. All the planets (more or less) stay near the plane of the Earth's orbit as they move around their orbits. The Sun's north pole projected onto the celestial sphere becomes the ecliptic north pole. The Earth's orbit corresponds to the ecliptic equator. As it turns out, if we choose the point where the plane of the Earth's equator intersects the ecliptic equator as the ecliptic's zero meridian, then conversions between these two system are much simpler. You might be surprised to learn that this hypothetical point has a very real location. It is precisely where the Sun will be at the instant Winter ends and Spring begins. That is why we call this meridian the Vernal Equinox. It is also called the first point of Aries even though over the centuries the Vernal Equinox has drifted out of Aries into Pisces.

In 1833 the Leonid meteor shower was so intense in the northeastern United States that a previously skeptical scientific community was forced to acknowledge the existence of rocks in space falling to the Earth. This shower occurs roughly every year. The Leonids are dust particles that were once part of Comet Temple-Tuttle. Sometimes the Leonid meteors are a great shower and sometimes they are a minor shower. There are reasons to hope that 1998 and 1999 will be unusually active years. Try looking for the meteors in the morning hours of November 17 and again on November 18th in 1998. In 1999, try November 18 and 19. On November 19th 1999 there are plans to launch a rocket from the old runway next to Frosty Drew Observatory to try to catch some comet dust. This comet dust will be analyzed by local area schools. The Westerly Sun had an account of this effort a few weeks ago.

Exciting as the rocket launch to collect comet dust will be next year, don't neglect to look for Saturn and Jupiter almost any evening. They are spectacular this Fall. A month ago, on a particularly fine evening, I sighted six satellites of Saturn at Frosty Drew, the best I have ever done with a seven inch telescope. Details on Jupiter and Saturn are very clear when the sky is stable. Fall is always my favorite time to view the skies.

Leslie Coleman
Leslie Coleman
Entry Date:
Oct 29, 1998
Published Under:
Leslie Coleman's Columns
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