Sky Roundup: October, 2007

Transit Authority

Astronomers mark the passage of objects in our sky with words that sound familiar but can have alternate meanings.

For example, when one is on the move from one place to another, we say that the individual is in transit. When a celestial object is on the move, it is also in transit. But its observation is usually in comparison to another celestial point of reference. One such reference often used is one’s own “fixed” meridian, a line projected against the sky for a person standing in a particular location. That reference line passes directly from north, through the zenith (that is, the top of the sky), to the south, and goes completely around the other side of the globe. Everyone has their own fixed meridian, depending on their location.

As the Earth rotates, the night sky seems to move, and, along with it, an entire grid of coordinates, namely, meridians of right ascension and parallels of declination. (Meridians of right ascension resemble lines of longitude projected onto the sky but are reckoned in hours of time, not degrees of angular measure; parallels of declination look and work just like lines of latitude projected onto the sky.) But one’s individual meridian, that is, the fixed meridian, remains stationary. So, when a star’s own meridian of right ascension coincides with one’s fixed meridian, that star is said to be making a transit.

The time of a particular transit is important to astronomers, for it tells the observer when the star is most favorably positioned (that is, highest in the sky) for observation. Knowing the time of transit of a particular star and sky coordinates of other stars can alert astronomers to the transit times of still more stars.

Similarly, a planet can be in transit. To the astronomer, the planet can make a transit when it, like a star, crosses one’s fixed meridian. Astronomers use the term more commonly to describe the passage of a planet in front of a star, such as the Sun. That is, a planet’s transit refers to the occasion when a planet, either Mercury or Venus, is seen moving across the disk of the Sun. These occasions are fairly rare (see Sky Roundup Archive, June 2007, for a recap of these transits) for Mercury and Venus.

Of course, on Earth we only see Mercury and Venus making transits across the disk of the Sun. From the vantage of Mars, the Earth itself makes transits across the Sun, though these are fairly rare. On a 284-year cycle, Earth transits as seen from Mars occur in intervals of 100.5, 79, 25.5 and 79 years. The last one occurred in May, 1984; the next one will occur in November, 2084.

And, according to astronomers, a satellite, viz., a moon, seen crossing in front of a planet also makes a transit. From our vantage on Earth, this may seem like a fairly unusual occurrence. In actuality, this can happen fairly frequently, and it is visible most often with the planet Jupiter. At times, the moon itself can be detected, superimposed on the disk of the planet; at other times, the shadow of the moon transits the disk of the planet. Of course one needs a telescope to see these transits.

More interestingly, this month and next is a series of double shadow transits that will be visible across the disk of Jupiter. That is, during the months of October and November, the shadows of two of the larger moons of Jupiter can be seen transiting the disk of Jupiter at the same time! Our almanac (below) lists the dates for shadow transits this month. There are five double shadow transits in October and eleven double shadow transits in November!

Transits of moons are known also to occur across the disk of Uranus. Considerably more distant than Jupiter, the disk of Uranus is extremely small in a good-sized telescope. But amateur and professional astronomers alike certainly do try to capture these rare events in images of that distant world. In fact, a moon of Uranus, viz., Titania, will be visible in a transit across the disk of Uranus on October 2-3, but one must have a reasonably large telescope, precise timing, and clear skies to view it successfully.

And planets can transit planets. If a small planet, like Mercury, moves in front of a larger planet, like Saturn, then that smaller planet transits the larger planet. (In his seminal work, Transits, published by Willmann-Bell, Inc., Jean Meeus explores planetary crossings of all kinds and from many vantages in the Solar System.) This actually happened in December 9, 1808, when Mercury made a transit across the disk of Saturn. In another example of planetary transits, Venus last transited Jupiter on January 3, 1818, and will do again on November 22, 2065.

However, if the angular size of the smaller planet in the foreground is greater than the larger planet in the background, then it’s called an occultation, as the smaller foreground planet hides the larger background planet. This last happened on July 21, 1793, when Mercury occulted Uranus. Mercury will occult Neptune on July 15, 2067.

Recall, too, that a transit is the passage of a planet in front of a star. If a star harbors a planetary companion, it may be detectable should it pass in front of its host star. Such planets orbiting other stars are called extrasolar planets, since they reside outside of our own Solar System.

Besides our Sun, various stars in the night sky have been found to be likely candidates for planetary transits. If a distant star hosts a large orbiting planet, astronomers can detect the planet as it passes in front of the star along our line of sight. The star’s light would dim minutely, but, depending on the size of the planet, that dimming of starlight is measurable. Some of these planetary transits are responsible for the variability detected in starlight, an aspect we explore at the Shafran Planetarium in our latest public sky show, “Variable Skies Tonight.”

For amateur astronomers, there are procedures and on-line programs available to help in the search for extrasolar planets. Even telescopes of modest aperture can join the search, which is fast becoming a hot trend in astronomical observing.

The cosmos remain a busy place, with stars, planets, and satellites all in transit.

Sky Roundup Almanac for Cleveland, September, 2007

Questions or comments?
Please contact Roy Kaelin at (216) 231-4600, x3405, or rkaelin@cmnh.org