The 2016 Transit of Mercury

the path of the 2006 transit of mercury across the sun

The path of the 2006 transit of mercury across the sun

On Monday, May 9, 2016, the planet Mercury will trek across the disk of the Sun beginning at 7:12 am EDT. This extraordinary astronomical event will be visible through telescopes using proper solar filtration and other methods of solar viewing. At The Franklin Institute, we will have our telescopes properly filtered for solar viewing to observe the 2016 Transit of Mercury.

What is a ‘Transit’?

According to NASA, a planetary transit is a special kind of eclipse and is defined as being “the passage of a planet across the Sun’s bright disk.” This phenomena is caused by an alignment of the orbit of our planet, Earth, with one of the two innermost planets of our solar system, Mercury and Venus.

Are Planetary Transits Rare?

Transits are extraordinarily rare events. There are usually only about 13 transits of Mercury per century and only transits of Mercury and Venus can be viewed from Earth. In order for transits to be visible, they need to happen during the daytime in the place where one is trying to view them from. For example, the last transit of Mercury occurred on November 8, 2006, but it was not visible in this region because it started just after sunset. After the 2016 Transit of Mercury, the next one will occur on November 11, 2019 and then there will not be another transit until 2032. The next transit of Venus will not occur until 2117.

Specifically, in respect to Mercury transits, former NASA Astrophysicist Fred Espenak, an expert on eclipses and transits, notes, “During the present era, transits of Mercury fall within several days of May 8 and November 10. Since Mercury's orbit is inclined seven degrees to Earth's, it intersects the ecliptic at two points or nodes, which cross the Sun each year on those dates.”

Espenak continues, “If Mercury passes through inferior conjunction at that time, a transit will occur. During November transits, Mercury is near perihelion and exhibits an apparent disk only 10 arc-seconds in diameter. By comparison, the planet is near aphelion during May transits and appears 12 arc-seconds across. However, the probability of a May transit is smaller by a factor of almost two. Mercury's slower orbital motion at aphelion makes it less likely to cross the node during the critical period. November transits recur at intervals of 7, 13, or 33 years while May transits recur only over the latter two intervals.”

How Can I View The 2016 Transit of Mercury?

Crowds of Franklin Institute members gather on the roof in the Joel N. Bloom Observatory to do some Sun gazing.

For the 2016 Transit of Mercury, the planet will appear as a small dot slowly moving across the Sun’s face from approximately the 10:00 position to the 1:00 position as we see our nearest star through a refractor telescope. Because Mercury is small, a minimum magnification of 50x is needed for observers to see the planet against the Sun’s face.

Just prior to Mercury’s first incursion onto the Sun’s disk, Mercury cannot be seen against the filtered black sky surrounding the Sun, nor can Mercury be seen just after it leaves the Sun’s disk. We’ll only see it as it crosses the Sun. The entire event will take 7 hours, 30 minutes for viewers in the northeastern United States.

The Franklin Institute will open at 9:30 am on Monday, and, weather permitting, we will be using solar filtered telescopes in our Joel N. Bloom Observatory to observe this event. The transit ends at 2:42 pm. Observation of the 2016 Transit of Mercury is free with museum admission, and, as of May 4, the weather forecast in Philadelphia looks favorable for viewing this event on Monday, May 9, 2016.

We hope you can join us!

Image: 2006 Transit of Mercury, NASA
Reference: Transits, NASA Eclipse Website