THE USM SOUTHWORTH PLANETARIUM

207-780-4249     www.usm.maine.edu/planet

70 Falmouth Street     Portland, Maine  04103

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.  

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.