Syzygy

In astronomy, objects can always cross in front of another object, if the alignments are right. They are usually referred to one of three terms: an eclipse, a transit, or an occultation. These three terms are part of a broader definition called a syzygy*.


   *Syzygy in astronomy is when three bodies are in a line.





Let's define the three of them.





Previously, I posted about eclipses of the Moon. These occur when the Moon, Sun, and the Earth are lined up in such a way that the Moon eclipses the Sun (a total solar eclipse), or the Moon goes through the Earth's shadow (a lunar eclipse). An eclipse can also happen in multiple star systems when one companion star passes in front of the other. What occurs is that the obscured body is either completed blocked out temporarily as it passes through the shadow of the eclipsing body (lunar eclipse) or the eclipsing body passes between the observer and the eclipsed object (total solar eclipse).





A transit is when a smaller body passes in front of a larger body, mostly a planet crossing in front of a star, but can also occur when a moon crosses in front of a planet, partially blocking out the Sun. Transits of extra-solar planets can be used to help astronomers find the planet and determine its size based on the light-curve of the star. Exoplanet transits are discussed more here. Transits occur in the inner solar system when Mercury and Venus cross in front of the Sun as seen from Earth. On my post about opposition and conjunction, what configuration(s) are Mercury and Venus in when they transit the Sun? Comment below if you know the answer. Also, the moons of Jupiter and Saturn can also transit across the face of their parent planets.

Released with Image The Galilean satellite Io floats above the cloudtops of Jupiter in this image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), two days after Cassini's closest approach. The image is deceiving: there are 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's clouds. Io is the size of our Moon, and Jupiter is very big.

Via NASA/JPL/University of Arizona

 

The last example of syzygy in astronomy is called occultation. In this case, the body that crosses between the observer and the more distant object appears much larger. These occur when the Moon, the Sun, or a planet pass in front of distant star, when the Moon passes in front of a planet, or when the satellite of a planet passes in front of an apparently smaller satellite.

Dione occulting Rhea (two moons of Saturn)

via NASA/Cassini-Huygens Mission

 

In picture form, this is what the three types of syzygy look like:

New Horizons

Today, July 14th, 2015 will go down as a major milestone in humanity exploration of the cosmos. After 85 years of pondering what Pluto actually looks like, we know and will learn more in the upcoming days and months.

New Horizons made its closest approach to Pluto at 11:50 UTC, allowing us to see it for the first time with clarity. By now, many of you have probably seen the images of Pluto with its heart-shaped surface feature, which was actually hinted at by Hubble images taken between 2002 and 2003.

If you look at the 180° face, a hint of the heart-shaped feature seen below may now be apparent.

This image of Pluto from New Horizons’ Long Range Reconnaissance Imager (LORRI) was received on July 8, and has been combined with lower-resolution color information from the Ralph instrument.

Via NASA/New Horizons

 

And from images taken on July 11, a composite of Pluto with its companion Charon.

A portrait from the final approach. Pluto and Charon display striking color and brightness contrast in this composite image from July 11, showing high-resolution black-and-white LORRI images.

Via NASA/New Horizons



For more information, follow New Horizons on Twitter and here, Alan Stern, Principle Investigator for New Horizons.

Also, visit the New Horizons page for updated images as they are posted.

 

Conjunction and Opposition

Sometimes, there are unique occurrences when planets align themselves with the Earth and the Sun. We actually have specific names for these occurrences: opposition and conjunction.

 

Opposition occurs when the Sun and the planet are in opposite directions in the sky, i.e. 180° apart in the sky. The Full Moon is also an example of opposition. Venus and Mercury, however, can never be in opposition. In my post about the morning and evening star, we know that Venus is never farther than 47.8° and Mercury is never more than 27.8° from the Sun.

 

Conjunctions happen to all the planets. The best way to think of it is when a planet and the Sun are in relatively the same direction in the sky. When the outer planets (Mars, Jupiter, Saturn, Uranus, and Neptune) are in the same direction as the Sun, we just refer to it as a conjunction. However, since Venus and Mercury can actually be at conjunction at two different points in their orbits, we need to specify their two conjunctions. When the inner planet is closest to the Earth, we call that its inferior conjunction because it is closer to the Earth than the Sun and when its at its furthest point, we call that the superior conjunction.

Planets can also be at conjunction with other planets. A good example is now visible in the night sky. Right now for a few days, Jupiter and Venus are near each other in the sky, and depending on your telescope or binoculars, are in the same field of view.

Venus and Jupiter in conjunction, with the full moon to give scale.

Via APOD

Composite Image Credit & Copyright: Wang, Letian