Transit of Mercury

On May 9, 2016, the planet Mercury will transit across the face of the Sun and for most of the Earth, the transit will be visible (or at least portions of it).


The transit begins at 11:12 UTC (Coordinated Universal Time measured at Greenwich, England) and ends at 18:42 UTC. To determine your time offset from UTC, Wikipedia has a good summary. For example, in Pittsburgh (where I live), we are currently only four hours behind UTC due to daylight savings time. So in Pittsburgh, the transit begins at 7:12 AM and ends at 2:42 PM.


To see the transit, you should not look directly at the Sun.. There are a couple of ways to look at it, however.

1. Have a Sun filter for a telescope, binoculars, or camera.
2. Watch it online at NASA.gov.

If you recall (or even if you don't), a transit is similar to an eclipse and an occultation. A transit is when a planet crosses in front of its parent star as seen from Earth. On Earth, only Mercury and Venus can transit the Sun. Unfortunately, many of us alive today will never witness another transit of Venus, as the last two took place in June of 2004 and June of 2012. The next transit of Venus will not take place until December of 2117. Mercury has a much shorter period of transits, approximately every three years, so if you miss this transit, you will only have to wait until November 2019.

General Relativity and Astronomy

Previously, we discussed how mass can curve space(time) due to general relativity. Why is this important?

The curvature of mass leads to interesting phenomena. The first is it causes the perihelion of a planet orbiting the Sun to precess. Secondly, it causes light actually to bend - yes, gravity affects light - and this leads to really weird stuff.

Let's look at the first one. The best example of the precession of a planet at it orbits the Sun is the path of Mercury. This was discussed back in the post about Mercury and General Relativity. The highlight of the discussion was that as Mercury orbits around the Sun, at perihelion, Mercury is in the deepest part of the gravity well created by the Sun. As it continues to orbit, each successive perihelion moves farther ahead in its orbit. The perihelion of Mercury was noticed in the mid 1800s, but was thought to be caused by an inner planet. But after Einstein's Theory of General Relativity was developed, the equations were able to show why Mercury's orbit precessed around the Sun. Everything that orbits around another body shows this precession, with the amount of precession dependent on the mass of the central body.

The second one seems a little weirder. From Newton's equation for universal gravitation, we see that the force of gravity is dependent on mass. However, light and all electromagnetic radiation are massless. So how does gravity bend light?

In a nutshell - gravity wells.

If a star were behind the Sun, in Newtonian gravity, we would not be able to see it, because gravity only affects objects with mass. We would see something like this.

However, because of General Relativity, light will bend in the presence of a gravitational field. The light from the star will curve around the Sun and we can see it from Earth.

This was actually proven by Sir Arthur Eddington. In 1919, there was a solar eclipse that he observed and photographed. When the pictures were analyzed, they could see the effect of gravity on stars. This analysis only worked during an eclipse because otherwise, the Sun would be too bright and wash out the background stars. Eddington gave physical proof that General Relativity was correct!

Positive and Negative Image of Solar Eclipse of May 1919

Image Credit:



F. W. Dyson, A. S. Eddington, and C. Davidson

This phenomena of light bending around masses also is used to search for exoplanets. As a planet passes in front of star, that planet can bend the light towards us. This process is called lensing. Not only does it allow the light to reach us even if the source is behind the mass, it can also magnify the light, making it brighter. Most of the gravitational lensing seen are galactic in nature.

The curved arcs are the lensed (background) object. The centers are the lenses bending the light.

Image Credit:

NASA/Hubble Space Telescope

Einstein's Cross (Gravitation Lensing) - Quasar being lensed by a central dim galaxy

Image Credit:

ESA/Hubble/NASA

Einstein Ring - When the background object is perfectly aligned with lensing object and the Earth, a complete ring can be created

Image Credit:

NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team

Hubble Site

The Morning Star and The Evening Star

Have you ever heard the terms "Morning Star" or "Evening Star"?  These terms are actually misnomers.  Neither object is actually a star.  They both refer to planets, either Venus or Mercury.  Today, we will answer why they are referred to either of these terms.

The ancients knew that both Mercury and Venus were close to the Sun.  They could only see them either in the morning or the evening.  This is how they got the name of Morning Star or Evening Star.  When they are the Morning Star, they rise just before the Sun and therefore, are only seen in the eastern sky.  When they are the Evening Star, they set just after the Sun and therefore, are only seen in the western sky.

If the ancients believed that everything in the solar system orbited around the Earth, how did they explain how Venus and Mercury stayed close to the Sun?  In the Ptolemaic model, the epicycles for both Mercury and Venus were connected to the Sun by a line.

Copernican Revolution



 

This line was needed in the model to keep Venus and Mercury close to the Sun in the sky.  Otherwise, in this model, Venus or Mercury and the Sun could be at opposition, which we know is not true.

 

When the Copernican Revolution occured and the heliocentric model came to prominence, the line was no longer needed.  Since Mercury and Venus orbit the Sun and in the model, are closer to the Sun than the Earth, their proximity to the Sun in the sky is easier to explain.

Copernican Revolution



We discussed the Phases of Venus in our last post and mentioned the greatest eastern elongation and the greatest western elongation.  For Mercury and Venus, we use these terms to tell us the point in Mercury's and Venus' orbits when they appear the farthest from the Sun in the sky depending on their position relative to the Sun.

Greatest eastern elongation occurs when Venus (or Mercury) is the farthest east of the Sun.  When either planet is at greatest eastern elongation, the planet is only visible from Earth just after sunset, which means we only see it when it’s in the western sky.  We call this the Evening Star.

Greatest western elongation, however, occurs when Venus (or Mercury) is the farthest west of the Sun.  When either planet is at greatest western elongation, the planet is only visible from Earth just before sunrise, which means we only see it when it’s in the eastern sky.  We call this the Morning Star.

Remember that the terms refer to the planet’s position with respect to the Sun in the sky, and not its location in the sky when it is visible.

By comparing the position of Mercury or Venus to the Sun, we can determing the angle between the planet and the Sun at either greatest elongation.  Venus cannot be any more than 47.8° from the Sun in the sky. At greatest western elongation, Venus rises about 2.5 hours before the Sun and at greatest eastern elongation, Venus sets about 2.5 hours after the Sun.  Mercury cannot be any more than 27.8° from the Sun in the sky.  At greatest western elongation, Mercury rises about 1.5 hours before the Sun and at greatest eastern elongation, Mercury sets about 1.5 hours after the Sun. 

Note that these elongations occur when the planet is at aphelion.  If either planet is at perihelion, the angle is smaller.  For Mercury, greatest elongation at perihelion is only 18° and is visible an hour before or after the Sun rises or sets.  For Venus, greatest elongation at perihelion is 45° which doesn't change its appearance before or after sunrise or sunset by much.

The reason for the huge difference for Mercury's elongations is its relatively elliptical orbit.  Venus' orbit is closer to circular so the angles are much closer.