Physics Department

Keeble Observatory

May 2003 Sky from the Keeble Observatory

May brings three solar system phenomena of particular interest - two eclipses and a transit. Understanding what these are and why we are able to see only one of them from Ashland requires only a little understanding of geometry.

Let's start with the transit of Mercury, during which the planet passes across the visible face of the Sun. This relatively rare event will take place on the 7th, but will happen about midnight in central Virginia. Your best chance of seeing it would be to take a trip to central Asia. The rarity of transits (Venus can also move across the solar disk) is explained by the size and orientation of Mercury's orbit. We know its orbital radius from the fact that Mercury is never more than 22.8 degrees east or west from the Sun. From the right triangle formed by Sun, Mercury, and Earth at these moments of greatest elongation, we can calculate that it is about 58 million kilometers from the Sun. (Earth is 150 million kilometers from the Sun, on average.) Observing it over time also tells us that its orbit is tilted 7 degrees from the plane of our orbit. Since the Sun is only ½ degree wide on the sky, the combination of orbital size and tilt means that most of the time Mercury will pass above or below the Sun as seen from Earth. However, Mercury crosses the plane of Earth's orbit approximately every 44 days. Inferior conjuction (when Mercury is closest to the Sun on the sky and also closest to Earth) occurs every 116 days. These two periods are in the ratio of 11:29 - which tells us that about every 29th plane crossing will also occur at every 11th inferior conjunction, and Mercury will transit of the solar disk. This is roughly every 3½ years.

A Full Moon happens when the Sun and Moon are on the opposite sides of the sky as seen from Earth. The Moon's orbit is tilted, carrying the Moon above and below the shadow cast by Earth. A lunar eclipse occurs when the Moon passes through that shadow, i.e. when the Moon crosses the ecliptic at a Full Moon. A solar eclipse occurs when the Moon crosses the ecliptic at New Moon. We'll experience both this month - but only the lunar eclipse will be visible from the Center of the Universe.

The underlying mathematics for determining the timing of lunar and solar eclipses is similar to that used for transits. However, since the periods involved are more nearly commensurate (the Moon's period relative to the position of the Sun is 29.53 days, relative to the stars it's 27.3 days), and because the Moon's orbit is tilted not just relative to the ecliptic but to the plane of Earth's equator, the cycle of eclipses is less regular. It takes between 18 and 19 years for the patterns to repeat.

On the 15th, the Moon rises a few minutes before 8:00 pm (EDT). It will penetrate the outer part of Earth's shadow (the penumbra) beginning at 9:05, and the dark inner shadow (the umbra) at 10:03. It will be entirely within the umbra for 26 minutes beginning at 11:14, not completely dark but glowing red from the light of myriad sunsets refracted through Earth's atmosphere. It will leave the umbra at 1:17 am, the penumbra at 2:14, and will set at 6:16 on the morning of the 16th.

You'll need no special viewing apparatus - take a lawn chair and some mosquito repellant and enjoy the show!

On the 31st there will be a solar eclipse. Because the Moon's orbit is an ellipse, at this particular occurrence the Moon will be farther than average from Earth, making it smaller in the sky. The eclipse will be annular - not completely covering the Sun, but leaving a "ring of fire" at mid-eclipse. Unfortunately, the eclipse will take place while the Sun is up for Iceland, not for Ashland!

Lunar phases for May: New on the 1st, at 8:16 am; First Quarter on the 9th, at 7:54 am; Full on the 15th, at 11:41 pm (total lunar eclipse); Last Quarter on the 22nd, at 8:32 pm. A second New Moon for this month occurs on the 31st, at 20 minutes after midnight (annular solar eclipse).

Morning planet watchers will still find Venus to the southeast at sunrise, moving closer to the Sun's position as the month advances. Mars is high to the south before sunrise, and Mercury will put in a predawn appearance beginning at mid-month, though it remains close to the horizon even at the end of the month.

Evening planet watchers will continue to see Jupiter and Saturn most of the month. Jupiter is still close to the "Beehive Cluster" - it's worth a look with binoculars, high to the southwest. Saturn is between Jupiter and the Sun's position, setting about two hours after sunset by the end of May. Mercury is lower to the west as the sun dips below the horizon on the 1st, but is moving rapidly into the glare of the Sun and may not be easily discerned. It will pass in front of the Sun on the 7th, but not at a time visible from Ashland - if you're traveling in Asia with a small telescope and appropriate solar filters, you'll have the chance to see this event.

Look overhead about an hour after sunset, you'll likely find that Jupiter is the first "star" to emerge from the twilight, brighter than all the real stars. Against the background stars, it hasn't moved much since last month, remaining between the Twins, Castor and Pollux in Gemini and bright Regulus in Leo. But, those background stars have rotated about 30 degrees to the west against the horizon, bringing new constellations into view and sending the familiar shape of Orion, below the horizon until fall. From Regulus we can trace eastward to Arcturus, in Bootes, and then southward to bright Spica in Virgo. This broad triangle marks the approach of summer.

The faint glow of the Milky Way - the diluted light of a hundred billion stars - arches slightly above the western horizon. By 10:00 it circles the horizon, an orientation it did not have last month until nearly midnight.

For your own monthly star chart, you can direct your web browser to http://www.skymaps.com. You will find extensive descriptions of what's worth looking for, and you can download and print a single copy for your personal use.

Copyright 2003
George Spagna