The Transit of Mercury
November 11th 2019
by Martin J. Powell
On November 11th 2019 a transit of Mercury across the face of the Sun took place. Mercury transits occur thirteen or fourteen times per century and they can be observed from much of the world, however since they involve viewing the Sun, extreme caution must be taken when attempting to observe them (for details on how to safely view the Sun, see below).
During a transit, Mercury is seen as a tiny black dot moving slowly across the Sun from Eastern limb to Western limb (from left to right in the Northern hemsiphere and right to left in the Southern). The 2019 transit commenced on November 11th at 12:35 UT (Universal Time, which is equivalent to Greenwich Mean Time) and ended later that same day at 18:04 UT, with mid-transit taking place at 15:19 UT. The total duration was therefore about 5½ hours, although due to the effect of parallax the exact duration varied by a couple of minutes depending upon the observer's location on Earth. In 2019 the planet crossed the Sun in a South-east to North-west direction, as opposed to a North-east to South-west direction at the previous transit in May 2016. This is because November and May transits of Mercury are viewed from opposite sides of the Earth's orbit, Mercury being seen ascending (moving from South to North of the ecliptic - the apparent path of the Sun against the background stars) during November transits and descending (moving North to South of the ecliptic) during May transits. After 2019, the next two Mercury transits will take place on November 13th 2032 and November 7th 2039.
NASA’s Solar Dynamics Observatory imaged Mercury when it last transitted the solar disk in May 2016 (click on the thumbnail for a larger version, 44 KB). This is a frame capture of the planet's egress, taken from a 2-minute video which can be seen here (Image: NASA Goddard Space Flight Center / Genna Duberstein)
Because Mercury is seen against the solar disk, a transit can be viewed from anywhere on the Earth where the Sun is above the horizon at the time of the event. The 2019 transit could be seen in its entirity from South America, most of Central America, the Caribbean, the Eastern USA and Canada, extreme Southern Greenland, extreme Western Africa, most of the Atlantic Ocean, the South-east Pacific Ocean and most of Antarctica (the Sun being above the horizon throughout). For much of the inhabited world, however, the transit was already in progress at sunrise or sunset so it was not seen in its entirity. India, for example, saw only the start of the transit (from Western Gujarat state) whilst New Zealand only saw the final hour or so. The event could not be seen at all from Australia, most of Russia, central, Southern and Eastern Asia (China, Japan, the Korean peninsula, the Philippines, Malaysia, Indonesia) and the Arctic because the Sun was below the horizon from these locations.
The German astronomer Johannes Kepler (1571-1630) was the first person to predict a Mercury transit event, although he did not observe one himself. His prediction of a transit on November 7th 1631 enabled the French mathematician and astronomer Pierre Gassendi (1592-1655) to observe it, in the year following Kepler's death.
Circumstances of Mercury Transits
Transits of Mercury can only take place whenever the planet is close to its ascending node or descending node (the points in its orbit where the planet crosses the ecliptic) heading Northwards or Southwards, respectively. For such an event to occur the planet must be close to inferior conjunction (positioned directly between the Earth and the Sun) and also be positioned within about 2° of one of these nodes. Because of the positioning of the nodes in relation to the Earth's orbit, in the present era transits can only take place in the second week of November (ascending node) or in the second week of May (descending node). The ascending and descending nodes of the planets are positioned exactly 180° apart - hence the six-month difference between these two dates. Because the node positions change very slowly over time, the dates of Mercury transits are drifting later in the calendar. In the 18th century, for example, transits took place between November 2nd and 12th, whereas in the 21st century they take place between November 7th and 14th.
November transits of Mercury repeat at intervals of 7, 13 and 46 years whilst those in May repeat at intervals of 13, 33 and 46 years. November transits outnumber May transits in the approximate ratio of 7:3. November transits are faster than May transits because Mercury is then close to perihelion (its closest point to the Sun) and its orbital speed is therefore faster (perihelion on this occasion took place on November 16th). Conversely, May transits are slower because the planet is then near aphelion (its furthest point from the Sun) so its orbital speed is slower. The 2019 transit lasted about two hours less than the previous one in May 2016; the track (chord) of Mercury also passing much closer to the Sun's centre (see diagram below). Although shorter than its equivalent May transits, the 2019 Mercury transit was the longest November transit of the nine which take place during the 21st century. In fact, it was the longest November transit since 1973 and before the year 2144.
Tracks of Mercury across the solar disk in 2016 and 2019 (click on thumbnail to see the full-size image, 10 KB). Click here (14 KB) to see the timings of each event, shown at hourly intervals in Universal Time (UT), which is equivalent to GMT.
Venus can also be involved in solar transits, since it also passes directly between the Earth and the Sun; this last occurred in June 2012 and will next occur over a century later, in December 2117. Mercury, however, appears much smaller than Venus when viewed from the Earth so optical aid is always required to view the event. Transits of Mercury are much more common than those of Venus - during the 21st century, for example, there are fourteen Mercury transits but only two Venus transits (in 2004 and 2012).
During the 2019 transit Mercury had an apparent diameter of 10" (10 arcseconds). The Sun's apparent diameter at this time of year is 32'.3 (i.e. 1938") which means that the Sun's apparent disk was approximately (1938 ÷ 10) = 193 times larger than that of Mercury (to gain a sense of just how small the planet appeared on the solar disk, see the scale diagram here, 65 KB). Because of parallax, Mercury's disk appeared very slightly displaced in a vertical sense on the solar disk when seen from locations in the far North and far South of the world. For example, an observer situated in Nuuk, Greenland saw Mercury positioned 19" further South on the Sun's disk than an observer situated at Punta Arenas, Chile - an angular distance which is equivalent to almost two apparent Mercury-diameters. Note that all of the diagrams on this page show Mercury positioned in a geocentric sense, i.e. from a theoretical position at the Earth's centre.
Although mid-transit occurred on November 11th at 15:19 UT, it did not coincide with the exact moment of Mercury's inferior conjunction - this took place some four minutes later. At mid-transit Mercury passed 75".9 (1'.2 or 0°.02) to the North of the Sun's centre; this was the planet's closest apparent passage to the Sun's centre since 1973 and before the year 2190.
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The Mercury Transit 2019
World Time/Visibility Map
The time/visibility map shows the changing events during the transit through a series of seventeen frames. As Mercury moves across the Sun's disk, the Earth's shadow moves accordingly across the world map and the local times across the world are displayed at each stage. It commences on November 11th at 12:35 UT (the start of the transit), after which there is a 25-minute interval to 13:00 UT. The next 5 hours 40 minutes (13:00 UT through to 17:40 UT) are shown at 20-minute intervals, after which there is a 24-minute interval, bringing the time to 18:04 UT (the end of the transit). The time/visibility map therefore allows one to estimate the times at which the transit was visible from anywhere in the world to within about 20 minutes.
The Transit of Mercury, November 11th 2019 Time/visibility map showing the local times and world visibility of the whole event (click on the buttons to view each frame and click on each frame for the full-size version [125 KB each]). The world map shows the regions of the world from which the transit was visible. In the lower half of the graphic, the time zones show the local times at which the event took place, listed by their military and local time zone designations.
A legend showing the local time zones and island designators used in the time/visibility map can be found in the following tables: time zones A to E** (26 KB), F to M (35 KB), M* to S (25 KB) and T to Z (21 KB). To determine the time zone in which you are situated, locate your Standard Time offset from Greenwich (hours East or West of Greenwich) shown in italics beneath each zone letter (alternatively, refer to the world map at TimeandDate.com). For example, the standard time at Los Angeles, USA is 8 hours behind Greenwich, so the relevant times will be found in the '-8 ' zone, i.e. time zone U ('Uniform'). For more details on how to use the animation for your own time zone, refer to the main text below.
The start of a transit (when Mercury moves on to the solar disk) is known as the ingress whilst the end (when Mercury moves off the solar disk) is known as the egress (see illustration below). Both ingress and egress are divided into exterior contact (where Mercury's disk is externally tangential to that of the Sun) and interior contact (where Mercury's disk is internally tangential to that of the Sun). Timings of the ingress, mid-transit and egress are given in the box at the upper left of the time/visibility map (times are shown in UT). Also shown is the position angle (P.A.) of Mercury on the solar disk, i.e. its compass bearing relative to the Sun's centre (measured anti-clockwise from North through East, South and West, where North = 0°, East = 90°, etc). Beneath the ingress and egress times the lower section of the Sun's disk is shown, with Mercury appearing as a tiny black dot (shown to scale). The world map beside it shows which regions were in daylight or night-time as the transit took place.
Diagram showing the Ingress and Egress stages of a solar transit (click on the thumbnail for a full-size image, 13 KB). The four stages of ingress and egress (i.e. from upper left to lower right in the diagram) are alternatively referred to as first (I), second (II), third (III) and fourth (IV) contacts respectively.
Beneath the illustrations, the time zones of the world are shown, each zone extending 15° in longitude (equivalent to one hour of the Earth's rotation). The zones are listed according to their military designations (where A = 'Alpha', B = 'Bravo', C = 'Charlie' etc). Under this scheme, time zone Z ('Zulu') represents Greenwich Mean Time; this zone is centred on the Greenwich Meridian (0° longitude). Time zones A to M are situated to the East of Greenwich (i.e. to the right of 'Zulu' time in the time/visibility map) whilst zones N to Y are to the West of Greenwich (to the left of 'Zulu' time in the time/visibility map). Because of space limitations, the time zones in both East and West regions are split into two, one above the other. For quicker identification, the approximate locations of each time zone is marked above and below the world map.
Each time zone column is headed by its military letter designation. Beneath it, in italics, is the standard time difference from Greenwich (i.e. not the summertime offset). Hence at New York, USA, the standard time difference from Greenwich is -5 hours (5 hours behind Greenwich) and it therefore falls under time zone R ('Romeo'). Beneath the italicised time offset is displayed the Standard Time operating in that zone; hence in zone R it will be 5 hours behind the Greenwich time. Since it was Spring in the Southern hemisphere, Daylight Savings Time operated from some locations here and wherever this applied it is shown at the bottom of the column. Hence in zone M, New Zealand Daylight Time (NZDT) was operating, which is one hour ahead of the Standard Time.
Within each military time zone column, up to six local time zones are shown. These are typically the six most populous local time zones within that military zone, listed in order of approximate latitude from North to South. Hence in military time zone D ('Delta') the local zones SAMT (Samara Time), GET (Georgia Time), Azerbaijan Time (AZT), AMT (Armenia Time), GST (Gulf Time) and MUT (Mauritius Time) are listed, these being the most populous local zones within that military time zone - SAMT (Western Russia) being furthest North in latitude and MUT (Mauritius) being furthest South. In some instances only one country operates a particular time zone; hence in time zone E** ('Echo star star') Nepal is the only country operating its time offset of +5:45 hours, so only the abbreviation NPT (Nepal Time) is listed. Where space allows, individual islands are listed, even where they operate on the same local time zone as their parent country.
The time zone lettering is colour-coded to indicate whether the transit was wholly visible, partly visible or not visible from any given military and/or local time zone (the military time zone letters above and below the world map also change colour accordingly). Because the Earth's shadow falls obliquely across the time zones at this time of the year, in many cases one region of a time zone could see the transit whilst another region could not, despite the clock times being the same; wherever this situation occurred, the time zone data appears orange. For example, from South Africa (military time zone B, local zone SAST) the start of the transit (at 14:35 SAST) was visible so the initial period is shown in yellow. By 18:20 SAST the Sun was setting over the North-eastern part of the country (Limpopo and Mpumalanga provinces) so the 'SAST' designator turns orange (i.e. it was not visible from all of this zone). By 19:40 SAST the Sun had set over the remainder of the country, so the designator turns grey and it remains so through to the end of the transit. The Sun/Mercury diagram (upper left of the time/visibility map) shows that when the North-eastern region of South Africa lost sight of the transit, Mercury was about two-thirds of the way across the solar disk (note that the diagram shows a Northern hemisphere view, with North upwards and East to the left). By the time the Sun had set over the Western coast (Northern and Western Cape provinces) the planet had traversed over 90% of its track across the Sun.
The transit was wholly visible from time zones O ('Oscar'), P* ('Papa star') and Q ('Quebec'), all in the Western hemisphere; consequently, these zones appear yellow throughout the time/visibility map. The transit was not visible from time zones E* ('Echo star'), F* ('Foxtrot star') and time zones G ('Golf') through to L ('Lima') - all in the Eastern hemisphere - since the transit took place during the local night time. These zones are therefore coloured grey for the duration of the time/visibility map. From all other time zones, the transit visibility changes between not visible, partly visible and/or wholly visible, depending upon where the region was situated in relation to the Earth's shadow at any particular time.
The zenith position of the Sun (where it was positioned directly overhead) is shown on the world map by the symbol . On November 11th 2019 at 15:19 UT the Sun's declination (angle from the celestial equator) was -17°.4, which meant that the Sun passed through the zenith around midday from all locations along latitude 17° 24' South (it passed overhead in central Brazil, central Bolivia and Southern Peru during the course of the transit). The zenith position of the Moon is likewise shown by the symbol .
The line (curve) dividing the light and dark regions of the world (or of any planet or moon, for that matter) is known as the terminator. On the world map it is labelled Sunrise on the shadow's Western edge and Sunset on the shadow's Eastern edge. It follows that the closer to the terminator a particular daylight region of the world is positioned, the lower in the sky the Sun will appear from that region.
At latitudes South of about 73° South the Sun was above the horizon throughout the day at this time of year, hence the entire transit was visible. Close to the Southern edge of the terminator, daylight lasts over 18 hours at this time. On the Banzare Coast in Wilkes Land, Antarctica, the Sun set with Mercury just entering the solar disk, then rose again 5½ hours later with the planet just moving off the disk. Conversely, along the Northern edge of the shadow, the daylight is relatively short at this time of the year. Residents of the coastal towns around Disko Bay in central Western Greenland (WGT, Time Zone P) saw the Sun rise with Mercury already in transit, the Sun setting some 4½ hours later with the planet still on the disk - the event therefore having been visible for the entire local daytime. Meanwhile residents occupying the Southern tip of Greenland saw the entire transit, the region being bathed in low-angle sunlight, whilst those in the Northern half of the country saw none of it, the area falling in shadow throughout.
Note that from a few time zones in the central Pacific Ocean the transit was partially visible, but not on November 11th local time. Time zones M ('Mike'), M* ('Mike star') and M** ('Mike star star'), which straddle the International Date Line, saw only the latter part of the transit just after sunrise on November 12th local time.
More precise transit times for various cities across the world can be found at the following websites:
The 'Black Drop Effect'
During a solar transit, both Mercury and Venus are subject to a mysterious phenomena known as the black drop effect. First observed during a transit of Venus in 1761, it occurs just after the moment of second contact (ingress, internal contact) and also just before the moment of third contact (egress, internal contact). Seen at high magnification, Mercury sometimes appears elongated in the direction of the Sun's limb, causing the planet to appear momentarily 'teardrop-shaped'. The apparent 'ligament' between the planet's limb and the Sun's limb appears greyish and fuzzy, often frustrating astronomers' attempts to determine the precise moment of second and third contact. The effect can be simulated by extending one's hand out against a bright background and then slowly bringing one's forefinger and thumb together; as the two fingers are about to touch, a fuzzy 'ligament' appears to join them together.
In the case of Venus, the scientific theory for many years was that the phenomena was caused by sunlight refracting through Venus' atmosphere in the direction of the Earth, thus distorting the planet's apparent shape. However, the high number of observations by amateurs and professionals during the planet's 2004 transit began to cast doubt on this explanation. Whilst smaller telescopes users were often seeing the 'black drop', many observers using larger instruments did not.
The 'Black Drop Effect' photographed by Scott MacNeill from an observatory on Rhode Island, USA, during the May 2016 transit of Mercury (click on the thumbnail for a larger image, 5 KB). Note the apparent 'ligament' connecting the planet's silhouette with the solar limb (Image: Scott MacNeill / Astronomy.com)
The mystery was mostly solved when astronomers studied the results of an Earth-orbiting satellite observation of a Mercury transit in 1999. Unlike Venus, Mercury does not have any significant atmosphere but the effect was nonetheless observed - hence the 'black drop' could not have been caused by atmospheric refraction. Neither did the Earth's atmosphere play any significant role (i.e. turbulence or poor seeing conditions) since the satellite observing Mercury was, of course, orbiting the Earth! The astronomers concluded that the 'black drop' was caused by a combination of two factors: a telescope's inherent optical defects (causing any image seen through it to be slightly blurry) and solar limb darkening, i.e. the darkening effect around the Sun's circumference caused by the gas in that region being more opaque - and therefore darker. These two factors combine to cause a dark blurriness at the point where the limbs of Mercury and the Sun touch - hence causing the 'black drop effect'.
Observing the Mercury Transit
The safest way to observe a transit of Mercury (or Venus) is to project the image of the Sun through a refracting telescope on to a piece of white card (the image of the Sun is projected backward through the telescope, from the main object glass through to the eyepiece and on to the card). In practice, a second piece of card is normally attached to the telescope, positioned just ahead of the eyepiece and perpendicular to the telescope's axis, in order to create a shadow around the projected image and thereby improving its contrast. The solar image on the card appears pale white, the silhouette of Mercury looking like a small black dot (rather like a sunspot). The projected image may be flipped horizontally and/or vertically, depending upon the telescope's optical arrangement. To prevent the risk of looking directly at the Sun, the telescope's finderscope should either be capped or removed completely. For refracting telescopes fitted with solar filters, a star diagonal eyepiece should be fitted so that the Sun can be safely observed by looking at right-angles to the telescope's main axis.
Binoculars can similarly be used to project the Sun on to a piece of card. The resulting image is naturally smaller than that from a telescope and, of course - unless one of the lenses is capped - the binoculars produce two identical images.
Methods of safely viewing the Sun Three techniques by which a transit of Mercury can be safely viewed (click on each image for a larger version, 60 KB / 50 KB / 59 KB): (Left) solar projection by telescope (Centre) projecting the Sun using a pair of binoculars and (Right) attaching an aluminised solar filter to the front of a telescope (Image sources: solar projection by telescope from Steve Ringwood / Astronomy Now; binocular projection by ThudsCave.com; telescope with solar filter by SpaceCentre.nz).
Transits can also be observed safely through a telescope by attaching an aluminised mylar solar filter to the front of the telescope, ahead of the object glass (always ensuring beforehand that the filter has not been damaged in any way!). If an aluminised solar filter is used the solar disk may appear pale blue, pale yellow or white, depending upon the manufacturer's specific design. Filters can also be purchased which can be attached to conventional cameras for photographing the event.
The apparent size of Mercury's disk during a transit - only one-fifth of that of Venus at its 2012 transit - is too small to be seen with the naked eye. Consequently, commercially-available filters allowing direct viewing of the Sun using a pair of cardboard 'spectacles' (solar viewers or eclipse shades) are of no use in observing a Mercury transit.
In 2016 the UK's Open University produced a short video describing how to safely view a Mercury transit. More information on how to safely view the Sun can be found at the Society for Popular Astronomy's website.
Internet Live Broadcasts
The following websites - among others - broadcast the 2019 transit live on the Internet:
Copyright Martin J Powell February 2019
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