"We are all in the gutter, but some of us are looking at the stars." -Oscar Wilde
THE DAILY ASTRONOMER
Wednesday, March 30, 2016
A Mercurian Transit Primer
About six weeks from now, the planet Mercury will appear to move across the Sun. Such direct "passages" of planets against their parent stars are known as "transits." Transits are noteworthy evens as they are certainly not common. We last observed a transit of Venus in 2012. The last transit of Mercury happened in 2006. Today's DA offers a brief "Transit of Mercury" primer that will hopefully prepare us to watch this upcoming spectacle!
We're using the conventional Q/A format for this primer. If you have a question we didn't include, please send it to me at [log in to unmask].
When will the next transit of Mercury occur?
May 9, 2016
How long will it last?
Seven and a half hours. It begins at 7:12 a.m. and ends at 2:42 p.m.
Will we eastern seaboard residents see all of this transit?
Yes. The transit starts after sunrise and ends well before sunset.
Will the transit be visible to the unaided eye?
No.
Mercury's apparent diameter is 1/158 that of the Sun. We recommend
that one use a telescope with at least a 50 x magnification in order to
observe this transit. NOTE: NEVER look at the Sun through a
telescope. Either use a filter or, preferably, project an image of the
Sun onto a wall and observe the transit in this image.
What will we actually see?
You will see a small black dot moving across the Sun. During this transit, Mercury moves "below" the Sun's center.
If I miss this transit, when will the next one occur?
November 11, 2019. Then, unfortunately, the following transit won't occur until November 13, 2032!
Why don't transits of Mercury happen more often?
Transits
of Mercury can only happen when Mercury is at inferior conjunction at
the same time it is at a node. A planet is in inferior conjunction when
it is between Earth and the Sun. (Only Mercury and Venus, the two
inferior planets, can ever be in inferior conjunction) A node is the
intersection point between two orbits. Mercury's orbit is inclined by
about seven degrees relative to Earth's orbit, generally called the
"ecliptic." Consequently, most of the time Mercury will either be "north" or "south" of
the Sun during inferior conjunction. Only when Mercury just happens to be at the intersection point during inferior conjunction will we see a transit. If Mercury's orbit and Earth's orbit were precisely aligned, a transit would occur during each inferior conjunction.
Can Mercury and Venus ever transit the Sun simultaneously?
They
can't at present because transits of Mercury occur only in either May
and November. Transits of Venus happen only in either June or
December. However, nodes shift around orbits so that these dates will
change. Eventually, Mercury and Venus will transit Sun
simultaneously. Or, more correctly, observers will be able to see both
planets against the Sun. This will happen in AD 69,163. A bit of a
wait.
Will the Sun's brightness diminish during the transit?
Theoretically,
yes, it will. However, the brightness reduction will be less than one
percent and won't be noticeable at all. Notably, astronomers can find planets around other stars by noticing how their brightness diminish when the planets move in front of them. This planet detection technique is known as the "transit method."