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THE DAILY ASTRONOMER
Monday, April 11, 2016
Mercurian Transit Module
We're attempting something new today, which should effectively pre-disaster our week so that the remaining days will likely proceed swimmingly. Today's DA will be the "Mercurian Transit Module" that we intend to distribute to the public. We will incorporate -or attempt to incorporate- all supplementary images into the e-mail. Should the images actually appear with the text, we can then make it a practise to include graphics and other visual aids with all subsequent Daily Astronomer articles, which, according to the famous prophecy we've occasionally cited, should numbers in the tens of thousands.
We did include the questions and answers we posted in recent DA articles, and for this repetition we apologize. However, we did add a couple questions not previously posted.
If you can't see the images in this e-mail, go to the DA web-page
http://usm.maine.edu/planet/da-7-december-2015
MERCURIAN TRANSIT MODULE
Monday, May 9, 2016
Transit series 7
TABLE OF CONTENTS
Introduction/Explanation
Time table
Reading the Transit Diagram Guide
Visibility regions
Questions/Answers
Transit Glossary
INTRODUCTION/EXPLANATION
On May 9, 2016, we will observe a transit of Mercury. During this event, Mercury will appear to pass directly across the Sun. The diagram below shows the "transit chord," or Mercury's path, for the May 9th event.
Mercury's path across the Sun during the 09 May 2016 transit. (Image: Courtesy NASA)
Mercurian transits can only occur when Mercury is at inferior conjunction when at or quite close to a node. A planet is in "inferior conjunction" when it passes between the Sun and Earth. A node is an intersection point between two planetary orbits. Mercury's orbit is inclined 7.01 degrees relative to Earth's orbit, also known as the "ecliptic." Consequently, Mercury will generally not be precisely aligned with the Sun and Earth during inferior conjunctions and no transit will occur.
This graphic shows both the ascending and descending nodes within the intersection of Mercury's orbit with the ecliptic. Transits of Mercury can only occur in May or November. May transits happen when Mercury is on or near the descending node. November transits occur when Mercury is on or near the ascending node. (Image courtesy: ESO)
When the transit begins, Mercury will be about 0.9 degrees S of the descending node, which Mercury will have passed 3.5 hours earlier. Mercury will continue its descent below the ecliptic during the transit, which explains the transit chord's southward trajectory. The transit will last almost seven and a half hours and will be entirely visible in the Greater Portland area. As Mercury appears quite small, Mercury will not be visible to the unaided eye during this transit.
This module will provide a time table specific to the local area, a guide to reading a "Transit of Mercury" diagram, an map indicating the visibility regions around the world, a list of questions and answers pertaining to the transit, and a glossary
contains terms related to transits.
TIME TABLE (PORTLAND, MAINE)
The Mercurian transit begins at 7:13:29 a.m. EDT in Portland, Maine. Times will hardly vary at all in other locations in Maine. (For more information about time differences in other locations, refer to the Questions/Answers section.) At this time, Mercury will have just "touched" the Sun's edge and we won't see anything, yet. However, within the next minute, one will observe a little "notch" appear as the planet begins its migration across the Sun.
SUN RISE (May 9): 5:22 a.m.
SUN SET (May 9): 7:53 p.m.
CONTACT I: 7:13:29 a.m. (Sun's altitude: 18.5o )
CONTACT II: 7:16:41 a.m. (Sun's altitude: 19.0o)
GREATEST TRANSIT: 10:57:46 a.m. (Sun's altitude: 56.5o)
CONTACT III : 2:38:05 p.m. (Sun's altitude: 53.6o)
CONTACT IV: 2:41:17 p.m. (Sun's altitude: 53.2o)
(Calculations courtesy of the US Naval Observatory.)
I: Contact I: when Mercury is externally tangent to the Sun. In other words, the moment just before Mercury first appears against the Sun. Contact I is also called the "external ingress" The transit begins. Relative to Earth's center, the transit begins at 11:12:19 UT (7:12:19 a.m. EDT) For Portland, Contact I begins at 7:13:29 a.m.
II: Contact II: the moment when all of Mercury first appears against the Sun. Or, the moment when Mercury is internally tangent to the Sun. Relative to Earth's center, Contact II occurs at 11:15:31 UT (7:15:31 EDT) For Portland, Contact II begins at 7:16:41 a.m.
GREATEST TRANSIT: the moment when the distance separating Mercury and the Sun's center is a minimum. Relative to Earth's centrer, the moment of greatest transit is 14:57:26 UT (10:57:26 EDT). For Portland, the moment of greatest transit is 10:57:46 a.m.
III: Contact III: the last moment when all of Mercury appears against the Sun. Or, the moment when Mercury when internally tangent to the Sun. Relative to Earth's center, Contact III occurs at 18:39:14 UT (2:39:14 EDT) For Portland, Contact III begins at 2:38:05 p.m.
IV: Contact IV: the planet's disk is externally tangent to the Sun. The transit ends. Relative to Earth's center, Contact IV occurs at 18:42:26 UT (2:42:26 EDT) For Portland, Contact IV begins at 2:41:17 p.m.
READING THE TRANSIT DIAGRAM GUIDE
This section is devoted to interpreting transit diagrams.
The aim of all diagrams is to contain an abundance of information within a very limited space. The diagram displays information pertaining to the May 9, 2016 transit of Mercury. NASA's Fred Espernak prepared this diagram. Below it one will find a key explaining each part of this graphic.
Greatest Transit: the moment when the distance between Mercury and the Sun's center is at a minimum
Geocentric Coordinates: coordinates relative to Earth's center. The coordinates change only slightly for observers on Earth's surface
R.A.: "Right Ascension" measures a body's distance from the point of the Vernal Equinox (first day of spring), which is currently located in the constellation Pisces. Right Ascension is measured in hours, minutes and seconds. The Sun's right ascension is 0 hours, 0 minutes, 0 seconds on the first moment of astronomical spring. At the moment of greatest transit, the Sun's center is displaced from the vernal equinox by 3 hours, 7 minutes, and 49.7 seconds and Mercury is displaces from the vernal equinox by 3 hours, 7 minutes, 59.5 seconds.
Dec: "Declination" measures a body's distance from the celestial equator, the projection of Earth's equator onto the sky. Declination is measured in degrees, seconds and arc-seconds. It ranges from +90 degrees declination for the north celestial pole to -90 degrees for the south celestial pole. On both the vernal and autumnal equinoxes, the Sun crosses the celestial equator and its declination is 0 degrees. The Sun reaches it greatest declination on the summer solstice, when it is about 23.4 degres north of the celestial equator. The Sun reaches its lowest declination on the winter solstice, when it is about 23.4 degrees south of the celestial equator. At the moment of greatest transit, the Sun's center is 17 degrees, 34' 38.4" north of the celestial equator, while Mercury is 17 degrees, 29' 52.6" north of the celestial equator.
S.D. "Semi-diameter" one half of a celestial body's angular diameter. Although the Sun's angular diameter changes throughout the year, as Earth's distance from the Sun varies continuously (see "perihelion" and "aphelion" in the glossary), its average value is about half a degree, or about 30'. Mercury's angular diameter is much smaller, as it is intrinsically much smaller than the Sun. At the moment of greatest transit, the Sun's semi-diameter is 15' 50.4", while Mercury's semi diameter is only 6.0"
H.P. "Horizontal parallax" The apparent difference in position of a body (as the sun, or a star) as seen from some point on the earth's surface, and as seen at Earth's center
UT: "Universal time" the time along the Prime Meridian, or at the Greenwich Royal Observatory. To convert from Universal Time (UT) to Eastern Daylight Savings Time (EDT), subtract 4. 11:13:29
I: Contact I: when Mercury is externally tangent to the Sun.
In other words, the moment just before Mercury first appears against the
Sun. Contact I is also called the "external ingress" The transit
begins. Relative to Earth's center, the transit
begins at 11:12:19 UT (7:12:19 a.m. EDT) For Portland, Contact I begins at 7:13:29 a.m.
II: Contact II: the moment when all of Mercury first appears against the Sun. Or, the moment when Mercury is internally tangent to the Sun. Relative to Earth's center, Contact II occurs at 11:15:31 UT (7:15:31 EDT) For Portland, Contact II begins at 7:16:41 a.m.
III: Contact III: the last moment when all of Mercury appears against the Sun. Or, the moment when Mercury when internally tangent to the Sun. Relative to Earth's center, Contact III occurs at 18:39:14 UT (2:39:14 EDT) For Portland, Contact III begins at 2:38:05 p.m.
IV: Contact IV: the planet's disk is externally tangent to the Sun. The transit ends. Relative to Earth's center, Contact IV occurs at 18:42:26 UT (2:42:26 EDT) For Portland, Contact IV begins at 2:41:16 p.m.
This diagram shows the position of a transiting planet during contact I, II, III and IV as well as the position of greatest transit, when the distance separating the planet and the Sun's center is at a minimum. Also shown is the position angle, the angle between the Sun's apparent north pole and the center of the planet at the moment of greatest transit. (Image courtesy of the Astronomical Almanac.)
Separation: the
apparent angular separation between Mercury and the Sun's center at the
moment of greatest transit. This value is measured in arc-seconds.
Position angle: the angle measured in degrees between the Sun's apparent North Pole and the center of the transiting planet at the moment of greatest transit. The position angle for Mercury is 153.8o at greatest transit.
Duration: the duration of the entire transit from Contact I - Contact IV.
VISIBILITY REGIONS
The world map above shows the regions where the Mercurian transit is wholly visible, partially visible, or not visible at all. The black dot shown in the Atlantic above South America indicates the position of the zenith Sun at the moment of greatest transit. (The zenith Sun is the place on Earth where the Sun appears directly overhead.) (Image courtesy: Fred Espenak, NASA)
We here in the Greater Portland area will see the entire transit of Mercury as the region is located within the area where the entire transit of visible. The entire transit will also be wholly visible in every part of world that is both east of the arc traversing middle America, central Canada, extreme eastern Mexico and Central America and west of the arc slicing across western Africa and through central Europe. In places such as Texas, California, and British Columbia, the transit will be in progress after sunrise. In countries such as Egypt, India, and Madagascar, the transit will still be in progress when the Sun sets. Australia, New Zealand, Indonesia and surrounding regions will not see any transit at all. Unlike total solar eclipses which are visible only within a narrow band, planetary transits are visible anywhere the Sun is above the horizon.
"TRANSIT OF MERCURY" QUESTIONS AND ANSWERS
How long will the May 9th transit of Mercury last?
Almost seven and a half hours. It begins at 7:13 a.m. and ends at 2:41 p.m.
Why does the transit begin and end at different times in different regions?
Because different observers will see different "transit chords." The path Mercury appears to follow across the Sun depends on the observer's location on Earth. As an example, let's imagine two people are watching a baseball game in a major league stadium. Both people are seated to the side of the pitcher, but one observer is on an upper deck and the other is at ground level. Both observers will see the ball moving from the pitcher toward the batter, but the upper deck observer will perceive it as moving against the field, while the ground level observer will perceive it as moving across the crowd on the other side of the stadium. Same event, but different perspectives. The proper term for this difference in perceived positions is "parallax." As far as the Mercurian transit is concerned, however, the difference is angle is very slight as Mercury will be about 51.9 million miles from Earth and the Sun will be approximately 93.9 million miles from Earth during the transit.
As examples, in Washington, D.C, the transit will begin at 7:13:36 a.m.
In Miami, Florida, the transit will begin at 7:13:52 a.m.
Will we eastern seaboard residents see all of this transit?
Yes. The transit starts after sunrise and ends well before sunset.
Will the transit be visible to the unaided eye?
No. Mercury's apparent diameter is 1/158 that of the Sun. We recommend that one use a telescope with at least a 50 x magnification in order to observe this transit. NOTE: NEVER look at the Sun through a telescope. Either use a filter or, preferably, project an image of the Sun onto a wall and observe the transit in this image.
What will we actually see?
You will see a small black dot moving across the Sun. During this transit, Mercury moves "below" the Sun's center.
If I miss this transit, when will the next one occur?
November 11, 2019. Then, unfortunately, the following transit won't occur until November 13, 2032!
When was the last transit of Mercury?
November 8, 2006. Almost a decade has elapsed since the last transit of Mercury. Fortunately, we'll only have to wait three and a half years for the next one.
Why don't transits of Mercury happen more often?
Transits of Mercury can only happen when Mercury is at inferior conjunction at the same time it is at a node. A planet is in inferior conjunction when it is between Earth and the Sun. (Only Mercury and Venus, the two inferior planets, can ever be in inferior conjunction) A node is the intersection point between two orbits. Mercury's orbit is inclined by about seven degrees relative to Earth's orbit, generally called the "ecliptic." Consequently, most of the time Mercury will either be "north" or "south" of the Sun during inferior conjunction. Only when Mercury just happens to be at the intersection point during inferior conjunction will we see a transit. If Mercury's orbit and Earth's orbit were precisely aligned, a transit would occur during each inferior conjunction.
Can Mercury and Venus ever transit the Sun simultaneously?
They can't at present because transits of Mercury occur only in either May and November. Transits of Venus happen only in either June or December. However, nodes shift around orbits so that these dates will change. Eventually, Mercury and Venus will transit Sun simultaneously. Or, more correctly, observers will be able to see both planets against the Sun. This will happen in AD 69,163. A bit of a wait.
Will the Sun's brightness diminish during the transit?
Theoretically, yes, it will. However, the brightness reduction will be less than one percent and won't be noticeable at all. Notably, astronomers can find planets around other stars by noticing how their brightness diminish when the planets move in front of them. This planet detection technique is known as the "transit method."
Is it true that Captain Cook observed both a transit of Venus and a transit of Mercury?
Yes. He observed a transit of Venus from Tahiti, in a location now known as "Point Venus." He also observed a transit of Mercury while in New Zealand from what is now known as "Mercury Bay."
What is the relation between a transit of Mercury and a former King of England?
A transit of Mercury occurred on May 3, 1661, the same date as Charles II's coronation. Charles II did not observe this transit, but famed astronomer Christiaan Huygens did see it.
When did people first see a transit of Mercury?
Johannes Kepler (1571-1630) was the first scientist to correctly predict upcoming Mercurian transits, but he never observed them himself. He actually predicted that a transit of Mercury would occur on November 7, 1631. The only person to observe this transit -and therefore the first person to have ever seen a Mercurian transit- was French philosopher/mathematician/astronomer Pierre Gassendi (1592-1655). Kepler died the previous year.
Why are transits of Mercury more common than transits of Venus?
Primarily because of the orbital paths. Mercury is closer to the Sun and therefore its orbit is much smaller than Venus. Consequently, Mercury moves faster than Venus and completes an orbit in much less time. Mercury needs about 88 days to revolve around the Sun. Venus requires 225 days. These times relate directly to the planets' "synodic periods." A synodic period is the time a planet requires to return to the same position relative to Earth. Mercury's synodic period is 116 days, which is the time that separates successive inferior conjunctions. Venus' synodic period is 584 days. Mercury will pass through inferior conjunction about three times a year, but more than a year and a half separates each inferior conjunction of Venus. Remember that a transit only occurs when the planet is at a node the same time it moves through inferior conjunction. Mercury is in inferior conjunction more often than Venus and its transits will be more frequent.
Transits of Mercury can only occur in either May or November, but I heard that November transits are more common. Is this true? If so, why?
We'll start by listing all 21st century transits of Mercury.
2003 May 07
2006 Nov 08
2016 May 09
2019 Nov 11
2032 Nov 13
2039 Nov 07
2049 May 07
2052 Nov 09
2062 May 10
2065 Nov 11
2078 Nov 14
2085 Nov 07
2095 May 08
2098 Nov 10
Five of the 21st century transits of Mercury happen in May. The other nine occur in November. So, yes, November transits are about twice as numerous as May transits. Between the years AD 1601-2300, inclusive, there will be 94 Mercurian transits. Of these, thirty-one will occur in May, and sixty-three will happen in November. To explain this difference, we need to discuss the concept of "Transit series."
We recall that transits only occur when an inferior planet is at a node during inferior conjunction. A node is an intersection point between two orbits. Two planetary orbits will actually have two nodes, ascending and descending. When Mercury crosses the "ascending node," it is moving "north" of Earth's orbit. When Mercury crosses the descending nodes," it is moving "south" of Earth's orbit. Transits can and will occur at both nodes. May transits occur when Mercury is at the descending node. November transits happen at the ascending node. A transit series consists of a sequence of transits happening at either node.
For example, Mercury transit series 8 consists of a sequence of November transits. The first occurred on November 2, 1776. That transit just "grazed" the southern tip of the Sun. The next transit in that series happened on November 5, 1822. During that transit, Mercury moved along a "higher" chord, though it was still "low." The following Series 8 transit was on November 6, 1868. Mercury's transit path on that date was even higher than the November 5, 1822 path. Since November transits occur when Mercury is at ascending node, each successive transit within a given series will migrate "northward" until the highest chord is reached, at which point the series ends. Mercury transit series 8 concludes on November 23, 2604.
Transit series 9 consists of May transits. The first such transit occurred on May 6, 1957. This transit path sliced across the northern tip of the Sun. The next transit in series 9 happened on May 7, 2003. Since May transits occur at the descending node, this transit path was "south" of the May 6, 1957 chord. The next series 9 transit -May 7, 2049- will be south of the May 7, 2003 transit chord. (Note: The transit on May 9, 2016 is part of series 7.) The Transit series 9 ends on May 16, 2371. This transit path will cut across a very small chord along the Sun's southern region.
Two graphics showing Transit Series 8 and Transit Series 9, the former of which consists of November transits and the latter of which consists of May transits. Successive transits within a given series are separated by 46 years. May transits occur when Mercury is on or near its descending node (see glossary). Consequently, each transit within a May series is shifted southward. Conversely, November transits occur at the ascending node (see glossary). As a result, each November series transit is shifted northward. (Images courtesy of Fred Espenak, NASA)
Now, what does all this have to do with the question as to why November transits are more frequent than May transits?
We have to introduce one more moving part to understand this lack of parity. A May transit occurs about a month after Mercury reaches aphelion, the point of greatest distance from the Sun. When any planet is at its aphelion, it is moving most slowly in its orbit. Conversely, when a planet is at its perhelion, its orbital velocity is at a maximum. November transits occur a few days before Mercury reaches perihelion and therefore it is moving most quickly.
We see that each transit in a series shifts either "northward" (November) or "southward" (May), because the transits happen at the ascending and descending nodes, respectively. The shift of Mercury's position relative to the Sun will be maximum (about 200 arc-seconds) when Mercury is moving most slowly, or in May. This shift will only be about 100 arc-seconds when Mercury is moving most rapidly, or in November.
So, if you miss this next May transit, you'll have to wait until 2049 to see another transit in the month of May.
Could someone could watch Earth transit the Sun from outer planets?
Yes,
Earth transits are sometimes visible from other planets. A while
ago, we withdrew this same question from Pandora and will be lazy sods
and repeat the answer:
The last transit of Earth from Mars occurred on May 11, 1984. The next
transit of Earth from Mars won't happen until November 10, 2084.
Also, and quite excitingly, the Moon will also appear to transit the
Sun, as well. It will either follow or precede
Earth across the Sun. If you miss the 2084 Earth transit event,
you'll have to wait until November 15, 2163. Transits of Earth from
Mars are presently only possible in May and November.
The following two transits of Earth from Mars occur on May 10, 2189 and then again on May 13, 2268.
Transits of Earth from Jupiter are more common. The last occurred on January 5, 2014. The next won't happen until January 20, 2026. Then, another occurs on June 24, 2055, followed by another on June 29, 2067.
The next transit of Earth from Saturn occurs on July 20, 2020. The next transit of Earth from Saturn after the 2020 event occurs on July 16, 2049.
Can planets 'transit" other planets?
Yes,
they can. These "transits" are called "transits" if the closer body
appears smaller than the more distant body so that both planets are
visible. The transits are called
"occultations" if the closer body completely blocks the more distant
planet. Such events are quite rare. The last planetary transit of
another planet happened on January 3, 1818, when Venus transited
Jupiter. The next planetary transit will occur November
22, 2065, when, yet again, Venus will transit Jupiter. The last
planetary occultation of another planet occurred when Mercury occulted
Uranus on July 21, 1793. The next planetary occultation of another
planet won't happen until July 15, 2067, when
Mercury will occult Neptune.
Can a transit of Mercury happen at the same time as a total solar eclipse?
Yes, but next simultaneous occurrence of the Mercurian transit and a total solar eclipse won't happen until August 20, 10,663! Note: Transits of Mercury can only occur in
May or November, presently. However, these dates shift so that eventually, Mercurian transits will happen in other months.
"TRANSIT OF MERCURY" GLOSSARY
APHELION
the point on a planetary orbit that is at the greatest distance from the Sun. Earth reaches aphelion around July 4th
ECLIPTIC
Earth's orbital plane around the Sun
EGRESSS
the period of time between Contact III and Contact IV (See "Reading the Transit Diagram" Guide")
GEOCENTRIC
relative to Earth's center. When astronomical event times are listed as "geocentric," corrections must be made for observers on Earth's surface, known as "topocentric."
INCLINATION
the angle between two orbits. Mercury's inclination angle relative to Earth's orbit is about seven degrees
INFERIOR CONJUNCTION
the passage of a planet between Earth and the Sun
INFERIOR PLANET
a planet that is closer to the Sun than a given planet. For Earth, the two inferior planets are Mercury and Venus
INGRESS
period of time between Contact I and Contact II (See "Reading the Transit Diagram" guide. )
NODE
the intersection points between two orbits. There are two nodes, "descending" and "ascending." When a planet reaches the descending node, it is moving "south" of the ecliptic. When a planet reaches the ascending node, it is moving "north of the ecliptic.
PERIHELION
the point in a planetary orbit that is at the least distance from the Sun. Earth reaches it perihelion around Jan 2 - 4.
TRANSIT
the direct passage of a planet across the Sun
TRANSIT SERIES
a sequence of transits that have a similar geometry. Successive transits within a given series are separated by 46 years. The May 9, 2016 transit is part of Transit Series 7. The next Mercurian transit within this series will happen on May 10, 2062.
ZENITH
point directly overhead
This "Transit of Mercury" module is a publication of the
Southworth Planetarium, University of Southern Maine
Dr. Jerry LaSala, Director
96 Falmouth Street
Portland, Maine 04103