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The Webfooted Astronomer - November 1999


Great Sky Events in November

by John Waters

This month gives us the possibility of a meteor storm and a transit of Mercury. So get out your telescopes, solar filters, lawn chairs, and hope for clear skies.

Mercury's Transit
On November 15 tiny Mercury will move across the face of the sun. Because the planet is only 1/194 the sun's apparent diameter, you'll need a telescope with a magnification of 50x or greater to observe the transit. Other visual and photographic requirements are identical to those for sunspots, the most important being a solar filter.

For safety purposes while observing the transit, always use a cover or solar filter over the aperture of your telescope. To prevent the optics and tube walls from heating, keep the filter on and the scope pointed in the direction of the sun, even when you're not observing. If you have a Newtonian or other design that permits it, you can experiment with how allowing air ventilation in the tube affects seeing.

Just like with a solar eclipse, there are four contact events during transit. Contact I occurs when Mercury’s limb first touches the sun. Contact II is the first instant when Mercury’s disk is fully visible on the sun. Contact 3 is the last instant when Mercury's disk is fully visible on the sun. Contact IV occurs when Mercury completely exits the sun's disk. Contact events 2 and 3 are probably the easiest to determine. These are when the thread of black between the planet and solar edge breaks or forms, respectively. (See the sidebar on page 4 for contact times in Seattle.)

If you are using a magnification that does not allow the whole solar disk to be seen, you may want to point you telescope at the spot where the transit starts. To help you get your directions in the eyepiece and thus find the north point on the sun, allow the sun to drift through your field of view. The movement of the sun is in the east to west direction.

Timing any of the contact events has scientific value, particularly in measuring the size of our ever-changing sun. Because poor seeing often increases the uncertainty in contact timings, you should make an estimate of the possible error associated with each timing. Send your observations and geographic coordinates (measured from a topographic map) to: Almanac Office, U. S. Naval Observatory, Washington, D. C. 20390.

If you miss this transit, don'’t worry. They happen about once every 8 years. But if you can’t wait that long I'll bet the Venusian transit in June 8, 2004, would be a fair substitute. Such transits happen in pairs separated by about 8 years approximately once a century.

Mercury Transit Times for Seattle
For Seattle the contact events are estimated to occur at the following times and the indicated number of degrees counterclockwise from the north point on the sun:

ContactLocal Time (PM)Degrees
2 1:22:48 21
3 1:59:13 18
4 2:10:35 17

Now that you know how to view the transit with your particular looking glass, let's consider some Alice-in-Wonderland-like facts about the planet itself. This will give you something to contemplate as you watch the transit and perhaps enrich your experience.

First, here are some typical planetary facts that may save you a trip to an encyclopedia. Mercury is about 1.4 times the diameter of the moon. Like our moon the surface is heavily cratered because it has essentially no atmosphere to protect it or cause surface erosion. The first recorded observation of Mercury was in 265 BC. The only spacecraft to visit it was Mariner 10 in the mid-1970s. Mercury is the only planet without some tilt in the axis of rotation. It has the second highest average density of any planet, about 98% that of Earth. It has the second most eccentric orbit of any planet, about 80% the eccentricity of Pluto. This causes the energy from sunlight at the surface to vary by almost a factor of two. At its maximum this energy is 10 times that received by Earth.

A while back a popular theory was that one side of Mercury always faced the sun due to solar tidal forces. But in the 1960s scientists discovered this was false when they were finally able to discern some features on the planet.

If you were a Mercurian, the sun would appear to move through your sky but much more slowly, finally completing one circuit in approximately 176 Earth days. Like on Earth the path of the sun would generally be from east to west but at times it would reverse itself. During this long Mercurian solar day, nearly half an Earth year, you would have experienced two rotations of your planet around the sun--two Mercurian years. Also during this time the star field would have rotated three times, giving you the experience of three stellar days. This unusual celestial behavior is due to tidal forces from the sun combined with an eccentric orbit.

With surface temperatures that range from about –300 to 800 F it's easy to imagine Mercurians would be interested in trying to beat the heat or cold. On any other planet, you can travel to an area where the sun shines, but isn't directly overhead. But a Mercurian has an added option Earthlings don't. Because of the synchronization of days and years, two broad longitudinal areas on the planet, 180 degrees apart, are permanently cooler, and two sections at 90 degrees to those are permanently hotter.

Leonids: Is This The Year of the Storm?
In November of each year, the Earth passes through the debris cloud from comet Tempel-Tuttle, creating the Leonid meteor shower. The meteor shower reaches a peak intensity every 33 years. In 1998 and 1999, the Earth is passing very close to the comet's orbital path at a time when the comet has recently passed. As a result, we are likely to experience a meteoroid "storm" as we pass through the cloud of debris. Rather than the 10 to 15 meteors per hour characteristic of a normal year, scientists predict we'll see 200 to 5,000 meteors per hour, and possibly more.

The point from where the Leonid meteors appear to radiate is located within the constellation Leo and is referred to as the radiant. The radiant is located in the western portion of that constellation in what is commonly referred to as the sickle or "backwards question mark." The radiant rises around 12:30 a.m. local time. Although a few Leonids can be observed prior to this, more will be seen after it rises. At about 3 a.m. the radiant is about 30 degrees above the horizon.

To best observe the Leonids wear appropriate clothing for the weather. Lay down in a reclining lawn chair with your feet pointing towards the east (the general direction of the radiant). Do not look directly at the radiant because meteors directly in front of you will not move much and you might miss fainter ones. Instead, keep your center of gaze about 30 or 40 degrees above or west of the radiant.

Last year the meteor shower peaked a full 16 hours before predicted, with several bright fireballs reported. Predictions are difficult because the meteor debris trail is not well mapped. This year, the shower is expected to peak on Wednesday, November 17, so it's best to observe every night from November 16–19.

Although the Leonids were first reported by Chinese astronomers in 902, the night of November 12-13, 1833, marks the true discovery of the Leonid meteor shower. During the hours following sunset on November 12, some astronomers noted an unusual number of meteors in the sky. But during the four hours that preceded dawn, the skies were lit up by a thousand meteors minute. Maybe it's our turn.

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