THE USM SOUTHWORTH PLANETARIUM
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43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458674.5
"There will come a time when you believe everything is
finished. Yet, that will be the beginning." -Louis L'Amour
THE DAILY ASTRONOMER
Wednesday, July 10, 2019
More Transit Queries
Four months from tomorrow (or 124 days from today), we'll see a transit of
Mercury; the last one visible from the eastern United States until May
2049! A transit, by the way, is the direct passage of a planet across the
Sun. From our perspective, only the two inferior planets, Mercury and
Venus, can ever transit the Sun. The next Mercurian transit will occur
on November 11, 2019. (The next Venusian transit is slated for December
10-11, 2117*).
As this will be the last such transit we'll observe for almost three
decades, we want to be sure to cover as many bases as possible! So, today,
we address some more questions pertaining to November's transit of Mercury.
*How much will the Sun's brightness diminish during the transit? Will we
notice any dimming or cooling?*
The transit of Mercury will diminish the Sun's brightness measurably, but
not noticeably. The brightness diminishment is calculable by taking a
ratio of Mercury's apparent area to the Sun's apparent area. The radius
of Mercury's disc from our perspective will be about 6 arc-seconds. The
Sun's will be about 955 arc-seconds. Square both values and then divide
the former by the latter and one will find that the Sun's brightness will
be reduced by 0.004% during the transit. Nothing to worry about at all: no
dimming or cooling. Granted, as it will be mid November, it will likely
be cool, anyway
*Why did we have a transit of Mercury in 2016, but will have to wait until
2049 for the next one? *
The minimum amount of time that can elapse between successive Mercurian
transits is 3.5 years. The maximum amount of time that can elapse between
successive Mercurian transits is about 13 years. This time, we just
happen to have two Mercurian transits occur within 3.5 years! (The May
2049 event will also be followed by another transit in November 2052.)
The next transit following the 11 November 2019 event won't occur until
November 2032, 13 years later, the maximum amount of time that can elapse
between successive Mercurian transits. Unfortunately, it won't be visible
in the eastern US because the Sun will be below the horizon during the
entire transit For the same reason, we'll miss the following transit in
November 2039. The next transit won't occur for another 9.5 years: May
7, 2049. Weather permitting, we will see that event!
*I heard that some people in the US will see all of the transit and some
others will see only some of it. Which of us will see it all?*
The map below shows the transit visibility regions throughout the world.
If you're in the eastern part of the United states, such as Maine,
Florida, or New York, you will see all of the transit as it will start
after sunrise. If you're farther west, the transit will be in progress
during sunrise. Texas residents will see almost all of the event.
Washington state observers will see less. The only people in the United
States who will miss out entirely are those who live in Alaska, save for
the southeastern most corner. For most Alaskans, the Sun will rise after
the transit ends. Residents in the southeastern region of Alaska will see
the very last part of the transit just after sunrise.
[image: tm2019-Fig02b.gif]
Most US residents will see at least part of the Mercurian transit.
Eastern US observers will see it all. Western US residents will observe
the transit in progress as the Sun rises. Only residents in most of
Alaska will miss the transit entirely, as the transit will end by the time
the Sun rises. Image: Fred Espenak
*Can Mercury and Venus ever transit the Sun simultaneously?*
Not now. Currently, Mercury can only transit the Sun in May and
November. Venus only transits the Sun in June or December. However, in
69,163 AD, Mercury and Venus will transit the Sun simultaneously. The
listed date for this simultaneous transit is July, but one shouldn't take
that specific date too seriously, as the difference between the dynamical
time (the time according to atomic clocks) and the civil time (clockwise
and calendrical) will be substantial after that time period, perhaps by as
much as four months. So, we know that Mercury and Venus will experience
the next simultaneous transit in 69,163, but we can't know the precise time
by our calendars. And, yes, that will likely be ok with most people.