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If you don’t have access to a white-
light or H-alpha fi lter-equipped tele-
scope, you can use an unfi ltered scope
with a low-power eyepiece to project an
image of the solar disk onto paper or a
screen. For ideas, see Jack Day’s design
for a projection system at https://is.gd/
SolarProjection. A fi ltered-but-direct
magnifi ed view will be better, though,
since Mercury is notably small: It’s just
4,880 km (3,030 miles) across, less than
half the size of Earth and only^11 / 119494 of
the Sun’s apparent diameter. In addi-
tion, even though Mercury’s at inferior
conjunction at the time of transit, it’s
also near perihelion, so it appears just
10 ′′ (arcseconds) across. That’s too
small to see without optical aid.
With a bit of magnifi cation, Mer-
cury’s orb is easy to fi nd, though. The
smallest planet appears distinctly disk-
like against the Sun, with a defi ned edge
and no penumbra. Hopefully, the Sun
will sport a few spots, as it did during
the 2016 transit. Dedicated viewers can
track the planet’s motion throughout
the day by comparing relative positions
of planet and sunspot, or planet and
Sun’s edge, which appears reasonably
defi ned through white-light fi lters.
Mercury makes its ingress (the
moment known as fi rst contact) onskyandtelescope.com • NOVEMBER 2019 49the east-southeastern rim of the Sun
at 12:35:27 UT. It takes 1 minute and
41 seconds for the trailing edge of the
diminutive dot to fully cross the border
and for the entire disk to be visible in
front of the Sun (at second contact).
As the moment of second contact
approaches, watch for the black-drop
effect, when Mercury appears as a small
black “teardrop” attached to the Sun’s
edge. This and other anomalous effects
can often be viewed in amateur tele-
scopes (S&T: May 2016, p. 38).
Mercury’s entire passage across
the Sun takes almost 5½ hours. Third
contact, when Mercury’s leading edge
touches the Sun’s west-northwestern
edge, occurs at 18:02:33 UT. Fourth con-
tact, when Mercury’s egress is complete,
occurs 1 minute and 41 seconds later.
The event is over.Citizen TOM & the A.U.
The distance from Earth to the Sun (the
astronomical unit, or a.u. for short) was
one of the most important measure-
ments in early modern astronomy. Once
scientists had that distance, the true
size and scale of the solar system be-
came clear. It wasn’t until Edmond Hal-
ley turned his attention to the problem
that anyone was able to fi gure out how
to measure the a.u. Halley determined
that during a transit of Venus, observers
at widely separated locations would see
Venus’s silhouette at different positions
against the Sun’s face due to paral-
lax. By measuring the shift in Venus’s
apparent position, astronomers could
calculate the mileage between Earth
and the Sun. Halley died before the next
transit of Venus, and so he never saw
his technique in action.
On November 11th, the Citizen TOM
(Transit of Mercury) Project will use Hal-
ley’s technique with a transit of Mercury
instead of Venus. Observers at multiple
sites across the country, using identi-
cal equipment (from the 2017 Citizen
CATE total solar eclipse project), will
take simultaneous images of the transit.
Most participants are students, offering
them a rare opportunity to make one of
the most important measurements in
astronomy from scratch.No
transit
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thSNEW17 h^ UT15 hUUUTTT
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midmidmidpoimidmidpoipopopo ntntnt
15:15:15:20 15:15:5: 20 20 2020 20 UTUTUTUTUEEcclliippttiiccccccccccccccccccccPathof^ Mercu
ryqAll of South America and parts of Central America, North America, and Africa will be able see the entire event, weather permitting. The rest of North
America, Africa, and Europe will see at least part of the transit. Asia, Oceania, and Australia are out of luck this time around.Mercury
moves onto
Sun 7:35 a.m.
ESTMercury
leaves Sun
1:04 p.m.
EST