CWL

Jupiter and the Moon Have a Close Encounter in the Sky February 18, 2013

The movement of the Moon makes a fascinating study of celestial mechanics. Despite the light pollution it brings to the nighttime sky, we’re fortunate as a species to have a large solitary satellite to give us lessons in “Celestial Mechanics 101″

This weekend, we’ll get to follow that motion as the Moon crosses into the constellation Taurus for a near-pass of the planet Jupiter, and for a very few citizens of our fair world, occults it.

In astronomy, the term “occultation” simply means that one astronomical body passes in front of another. The term has its hoary roots in astronomy’s ancient past; just like the modern day science of chemistry sprung from the pseudo-science of alchemy, astronomy was once intertwined with the arcane practice of astrology, although the two have long since parted ways. When I use the term “occultation” around my non-space geek friends, (I do have a few!) I never fail to get a funny look, as if I just confirmed every wacky suspicion that they ever had about us backyard astronomers…

But those of us who follow lunar occultations never miss a chance to observe one. You’ll actually get to see the motion of the Moon as it moves against the background planet or star, covering it up abruptly. The Moon actually moves about 12° degrees across the sky per 24 hour period.

On the evening of Monday, February 18th, the 56% illuminated waxing gibbous Moon will occult Jupiter for Tasmania and southern Australia around 12:00 Universal Time (UT). Folks along the same longitude as Australia (i.e., eastern Asia) will see a close pass of the pair. For North America, we’ll see the Moon approach Jupiter and Aldebaran of February 17th (the night of the Virtual Star Party) and the Moon appear past the pair after dusk on the 18th.

What is an Occultation?

An occultation occurs when an apparently larger body passes in front of an apparently smaller one.

Image: Conjunction and Occultation of Jupiter and the Crescent Moon

A transit occurs when an apparently smaller body passes in front of an apparently larger one. In the combined case where the smaller body regularly transits the larger object, an occultation is also termed a secondary eclipse.

An eclipse occurs when a body disappears or partially disappears from view, either by an occultation, as with a solar eclipse, or by passing into the shadow of another body, as with a lunar eclipse (thus both are listed on NASA’s eclipse page).

Transits, occultations and eclipses in general are all effects of a syzygy.

Transits and occultations of the sun by Earth’s moon are called solar eclipses regardless of whether the sun is completely or partially covered. By extension, transits of the sun by a satellite of a planet may also be called eclipses, as with the transits of Phobos and Deimos shown here, as may the passage of a satellite into the planet’s shadow, as with this eclipse of Phobos.

The term eclipse is also used more generally for bodies passing in front of one another. For example, this entry from NASA’s Astronomy Picture of the Day refers to the Moon eclipsing and occulting Saturn interchangeably.

Occultation of Mars

Venus Occultation in Motion

Timelapse video: Occultation of Venus by the crescent Moon in the morning sky of Korea. Rising in the dark hours before dawn, wandering Venus as the morning star is captured in this photo sequence as it hides behind the moon.

Venus - Lunar Occultation

Conjunction and Occultation of Jupiter and the Crescent Moon

Jupiter as it begins its occultation behind the waning crescent. Io and Europa are trailing and soon disappeared behind our lone Moon. Mare Crisium, on the Moon’s eastern quadrant, is the lunar surface feature nearest Jupiter.

Occultation of Zeta Tauri

Summary and Image: Mohamed Laaifat

The photo sequence above shows the waxing crescent Moon occulting the star Zeta Tauri. It was taken from Normandy, France on April 25, 2012, at about 10:30 p.m. local time.

This modest, third-magnitude star in the constellation of Taurus the Bull was occulted from our perspective here on Earth by the non-illuminated portion of the Moon. Because glare is minimal with an occultation such as this, an observer can more easily view the event and time the star’s disappearance and reappearance.

Note that Zeta Tauri appears the same when in contact with the lunar limb as it does in the other frames. If the Moon had an atmosphere, a slight difference in brightness could be detected.

Venus Occultation Behind The Fading Moon

Copyright: Emil Ivanov