THE 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: 2458730.5
2019-2020: II
"Kites rise highest against the wind, not with it."
-Winston Churchill
THE DAILY ASTRONOMER
Wednesday, September 4, 2019
Wednesday, September 4, 2019
Ten Transit Talking Points
68 days!
That is the time that now separates us from November 11, 2019, the date of the next Mercurian transit: the passage of Mercury directly across the Sun. We assure you that we will focus on many other astronomical matters in the meantime. However, this Mercurian transit is arguably 2019's most important celestial event. It will also be the last Mercurian transit we'll see here in the eastern US until May 7, 2049*. For these reasons, we consider important enough to warrant a few more DA articles, which won't be consecutive.
Today, we'll focus on ten basic transit talking points.
- 1. It won't be visible without a telescope or binoculars
Mercury is so far from us that it's angular diameter from our perspective never exceeds 13" (arc-seconds.) The first planet therefore is too small to be visible to us against the Sun and so a telescope or binoculars is needed to observe the transit. Of course, special masking is necessary whenever you're observing the Sun through a telescope.
-2. Hey! We see Mercury at night sometimes! Why isn't it too small to see then?
We see Mercury at night because it is reflecting sunlight and appears as a pinpoint light source. We're seeing this reflected light. When Mercury appears against the Sun, we're seeing its shadow which is too small to be observable without optical equipment.
-3. What is the time table?
The transit will begin at 7:36 a.m. Mid transit occurs at 10:20 a.m. and the transit ends at 1:04 p.m. for a transit duration of 5 hours and 28 minutes. The times will vary only slightly in other New England and eastern American locations. Do note that the transit beginning time refers to first contact. Mercury first appears to touch the Sun's edge at 7:36 a.m. The entire planet be visible against the Sun about 1 - 2 minutes later.
-4. It will not darken our skies
Total solar eclipses -1678 days until our next total solar eclipse!!- darken our skies for awhile during and around the time of totality. Transits of Mercury and Venus reduce the Sun's light negligibly. We won't notice any darkening
-5. Mercurian transits occur in May or November
If someone invites you to go to a remote valley to observe a transit of Mercury on Christmas morning, be wary. Transits of Mercury can only happen currently in May or November. Now, if someone invites you to go to a remote valley to watch a transit of Venus on Christmas morning, still be suspect. While transits of Venus do occur in either June or December, the next Christmas transit of Venus won't happen until AD 3818.
-6. Mercurian transits are more frequent than Venusian transits
Transits of Mercury or Venus can only happen when either planet is at inferior conjunction (between Earth and the Sun) around the same time the planet is at or near a node, the intersection point between the ecliptic and the planet's orbit. Inferior conjunctions of Mercury occur -on average- about every 116 days, while inferior conjunctions of Venus happen every 584 days. As Mercury is at inferior conjunction much more often than Venus, transits of Mercury are more frequent. About 13 - 14 transits of Mercury occur every year. Transits of Venus happen in eight year pairs separated by more than a century. The next transit of Venus occurs on December 11, 2117.
-7. The transit of Mercury in 1631 was the first transit to be observed.
Pierre Gassendi (1592-1655) is, unfortunately, not well known today. He was a renaissance man who was a priest, philosopher, astronomer and mathematician. He was the only person to have observed the 1631 transit of Mercury, one that Johannes Kepler correctly predicted. Kepler, himself, didn't see this transit as he died the previous year. Gassendi has the distinction being the first person to observe a transit.
-8. The least amount of time separating successive transits of Mercury is 3.5 years
The previous transit of Mercury occurred in May 2016. We were fortunate enough to watch this transit and will, weather permitting, see the next one this coming November. Three and a half years is the least amount of time that can elapse between successive transits.
-9. The greatest amount of time separating successive transits of Mercury is 13 years
The next Mercurian transit after the November 2019 event occurs in November 2032, thirteen years later, the greatest amount of time that can elapse between successive transits. The following transit happens in November 2039 and then the following occurs 3.5 years later in May 2042.
-10. Transits occurs in 46-year series.
A transit of Mercury series refers to a sequence of transits, each of which is separated by almost exactly 46 years. After this period of time, a transit will recur with nearly the same type of path around the same time of year. This repetition occurs because 46 revolutions of Earth is approximately equal to 191 revolutions of Mercury. So, after 46 years, Earth and Mercury will almost occupy the same positions relative to each other. The November 11, 2019 transit is part of series 247. The previous transit in this series happened on November 10, 1973; the next one occurs on November 11, 2065.
*The next two transits of Mercury occur on November 13, 2032 and November 7, 2039. Both transits will occur when the Sun is below the horizon from our perspective.
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