Sky and Telescope - May 2016 - 49

Learn how to use detailed star charts to find the faintest things
that your telescope can show: skypub.com/charts.

Alan MacRobert

Most of the inhabited
world will be able to
see at least part of
the transit, weather
permitting. Observers in eastern North
America and western
Europe can watch
both the entry and
exit of Mercury across
the Sun's edges.

Entire
Entire
transit
transit
visible
visible

Transit
Transit
in progress
progress
in
at sunrise
sunrise
at

Transit
Transit
in progress
progress
in
at sunset
sunset
at

No
transit
visible

Transit of Mercury
May 9, 2016

FRED ESPENAK

Eclip

If you're in the Americas, don't miss the chance
to watch tiny Mercury slip off the Sun. Be
watching by about 18:35 UT (2:35 p.m. Eastern
Daylight Time). Can you see anything of the
black drop effect as Mercury begins to contact
the Sun's limb?

Mercury
moves
onto Sun
7:12 a.m. EDT

E
12 h UT

tic

Path of

W

Mercur

15 h UT

Transit
midpoint
14:58 UT

y
18 h UT
Mercury
leaves Sun
2:42 p.m. EDT

S

And it moves! The most interesting aspects to watch
will be Mercury making its entrance and/or exit across
the Sun's limb. The planet will take 3 minutes and 12
seconds to do so. If you can watch at the time of ingress,
keep your eye on the limb barely south of due east for the
first detectable sign of a tiny dent. Use high power. You
can tell which limb is celestial east by turning off your
telescope's drive if it has one; the Sun will drift across
the eyepiece view from east to west.
At second contact - when Mercury's trailing edge
comes onto the Sun - watch for any sign of the black
drop effect: the illusory appearance of a tiny black line still
connecting the planet to the outer darkness. And does
the black disk show a central point of light? More about
such anomalous appearances begins on page 38.
As Mercury travels across the Sun's vast expanse, how
readily can you see its motion? If it passes near a sunspot, can you see that it's darker than even the sunspot's
umbra? When Mercury departs at egress, the sequence

of phenomena at ingress unwinds in reverse order.
Although transits of Mercury are less dramatic than
those of Venus, they come more often. The last two
Venus transits happened in 2004 and 2012 after a gap
since 1882, and not until 2117 will the world see another.
But Mercury passes between the Earth and Sun about 13
or 14 times every century. It will next do so on November
11, 2019 - again visible from the Americas and Europe.
There are a couple of reasons for the difference.
Mercury rounds the Sun more frequently than Venus
does and passes through inferior conjunction five times
as often. And Mercury is also closer to the Sun, so from
Mercury, the Sun presents a larger target for a line of
sight from Earth through the planet to hit.
As you're watching the transit, imagine a copy of
Earth replacing Mercury. It would look only 2.6 times
wider than the tiny dot - a reminder of how insignificant the terrestrial worlds appear next to the awesome
scale of our home star.
Sk yandTelescope.com May 2016

49


http://www.skypub.com/charts http://www.SkyandTelescope.com

Sky and Telescope - May 2016

Table of Contents for the Digital Edition of Sky and Telescope - May 2016

Contents
Sky and Telescope - May 2016 - Cover1
Sky and Telescope - May 2016 - Cover2
Sky and Telescope - May 2016 - 1
Sky and Telescope - May 2016 - Contents
Sky and Telescope - May 2016 - 3
Sky and Telescope - May 2016 - A
Sky and Telescope - May 2016 - B
Sky and Telescope - May 2016 - 4
Sky and Telescope - May 2016 - 5
Sky and Telescope - May 2016 - 6
Sky and Telescope - May 2016 - 7
Sky and Telescope - May 2016 - 8
Sky and Telescope - May 2016 - 9
Sky and Telescope - May 2016 - 10
Sky and Telescope - May 2016 - 11
Sky and Telescope - May 2016 - 12
Sky and Telescope - May 2016 - 13
Sky and Telescope - May 2016 - 14
Sky and Telescope - May 2016 - 15
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Sky and Telescope - May 2016 - 17
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Sky and Telescope - May 2016 - 21
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Sky and Telescope - May 2016 - 26
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Sky and Telescope - May 2016 - 28
Sky and Telescope - May 2016 - 29
Sky and Telescope - May 2016 - 30
Sky and Telescope - May 2016 - 31
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Sky and Telescope - May 2016 - 33
Sky and Telescope - May 2016 - 34
Sky and Telescope - May 2016 - 35
Sky and Telescope - May 2016 - 36
Sky and Telescope - May 2016 - 37
Sky and Telescope - May 2016 - 38
Sky and Telescope - May 2016 - 39
Sky and Telescope - May 2016 - 40
Sky and Telescope - May 2016 - 41
Sky and Telescope - May 2016 - 42
Sky and Telescope - May 2016 - 43
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Sky and Telescope - May 2016 - 46
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Sky and Telescope - May 2016 - 48
Sky and Telescope - May 2016 - 49
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Sky and Telescope - May 2016 - 84
Sky and Telescope - May 2016 - Cover3
Sky and Telescope - May 2016 - Cover4
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