On Monday 11th November, Mercury will transit across the face of the Sun. This planetary alignment will occur 14 times throughout the 21st Century, but, due to the size of Mercury and the time of year, observing this tiny black disk crossing the face of the Sun, can be challenging.
2019 Mercury Transit Facts
Mercury will take nearly 5.5 hours to cross in front of the sun. The transit will be widely visible from most of the Earth, including the Americas, the Atlantic and Pacific oceans, New Zealand, Europe, Africa and western Asia. It will not be visible from central and eastern Asia, Japan, Indonesia and Australia.
The transit of Mercury across the Sun will begin at 12:35 GMT on 11th November 2019 and finish at 18:04.
|12.35pm||Transit of Mercury begins||Mercury’s outline first touches the Sun’s edge|
|12.37pm||Full transit begins||Mercury fully enters the Sun’s disc|
|3.19pm||Mercury at Sun’s centre||Mercury is closest to the Sun’s apparent centre|
|4.17pm||Sunset in London||Transit continues but will not be visible in UK|
|6.04pm||Transit of Mercury ends||Not visible in the UK|
Johannes Kepler, a German astronomer, was the first to predict that a Mercury transit would occur. He calculated that both a transit of Mercury and a transit of Venus would take place in 1631, and issued a notice to fellow astronomers in 1629 asking them to observe the phenomenon.
Kepler died before he could confirm his predictions, but they proved to be accurate. On 7th November 1631, French astronomer Pierre Gassendi became one of the first people to witness a transit of Mercury.
Future observations of the transit would prove just as important. On 7th November 1677, Edmund Halley, watched the transit of Mercury from St. Helena, and later realised that transit observations would be instrumental in measuring the distance between the Earth and the Sun.
During Captain James Cook’s expeditions, his English astronomer companion, Charles Green, also observed the transit of Mercury on 9th November 1769 from New Zealand. A historic moment in time is coming too, when, on 12th November (in New Zealand), when almost 250 years to the day, astronomers will gather at Mercury Bay, New Zealand, to watch the transit of Mercury, where Captain Cook’s astronomer first observed this rare astronomical event.
Transits of Mercury this century can happen between 7th-10th May and 7th-14th November. The November transits occur about twice as often as May ones. This is because during a May transit, Mercury is close to aphelion – furthest away from the Sun – whereas during a November transit, it is closest to the Sun, at perihelion. This also makes Mercury’s apparent size smaller when viewing.
If you remember, in 2004 and 2012, there was a pair of Venus transits, but the next transit of Venus won’t be until 2117 and that won’t be visible from the UK, the next visible transit will be 2125. Now back to Mercury…
How to watch the transit of Mercury
Looking directly at the Sun is not recommended and could lead to permanent eye damage. Always use safe procedures when viewing the Sun.
A number of strict rules should always be adhered to when observing the Sun.
- When using a solar filter, always place the filter on the sun facing end of the telescope as the focused Sun will melt filters at the eyepiece end in a matter of seconds.
- If stopping down a large aperture telescope, make sure that the stopped down area is not receiving any stray light and blocks the Sun completely.
- Make sure that the filter cannot be blown off by gusts of wind – use some masking tape to secure the filter in place.
- Never leave a telescope pointing at the Sun unattended.
- Always check the solar filter is in place before you look through the eyepiece.
- Never look at the Sun through an unfiltered telescope using eclipse shades or a welder’s filter. And don’t use a camera with a telephoto lens, even if the lens has photographic filters on it that appear to darken the Sun.
- Filters that are not safe, though they are sometimes recommended in old books, include smoked glass, stacked sunglasses, crossed polarizing filters, and neutral-density camera filters. While these may greatly dim the Sun’s glare, invisible radiation may get through and damage your eyes. (Extract from the NIAAS website)
There are a number of safe ways to look at the Sun, including:
- solar glasses (not recommended)
- a telescope
Be aware that looking at the Sun can cause permanent blindness and needs care and attention to do safely. Children should be supervised by adults at all times.
You can project the image of the Sun using a refractor telescope, one side of a pair of binoculars or a monocular. NEVER LOOK AT THE SUN DIRECTLY THROUGH AN UNFILTERED TELESCOPE, LENS or CAMERA.
To project the image of the Sun safely, set up your refractor telescope with eyepiece or binoculars on a tripod in a clear level location. Observe the shadow that the circular shape of the instrument makes on the ground and point the telescope in the direction of the Sun. Using a white piece of card, hold the card in front of the eyepiece and use the focus ring to achieve a sharp image. Start off with a wide angle eyepiece, e.g. 40mm. before zoning in with a higher magnification.
If you want to use a pair of binoculars, it is best to cover one of the lenses.
There are a number of different types ranging from a few pounds to over one hundred pounds depending on the type you want.
For those with a telescope, you can make a relatively cheap solar filter using Baader Solar Film which comes in an A4 sheet which you then have to cut and glue onto a homemade lens cap. Instructions here. You should ALWAYS place the solar filter securely in front of the telescope, never at the eyepiece.
If you don’t have time to make a filter, you can purchase one made of solar film below.
Or for something a bit better quality, have a look for “Thousand Oaks” solar filters which are glass solar filters specially designed for telescope observing and photography.
If you are using a digital SLR, you can buy a Neutral Density filter, ND-100,000 which is specialised for solar photography. Do not be tempted to buy ND-1000 and build them up as the heat generated could be catastrophic.
Or, for a cheaper option for digital SLRs, you can buy a Cokin filter adapter and some Baader Solar Film and glue the film onto the flat surface of the Cokin adapter.
What will I see?
Those who pay close attention to Mercury’s transit can witness some interesting details.
Time-lapse photos or videos, for example, will reveal that Mercury moves in a curved line over the sun, and the planet’s path will vary according to your location on Earth. In the Northern Hemisphere, Mercury will appear at the bottom left section of the Sun, curving upwards towards the right and then tapering off, while in the Southern Hemisphere, Mercury will first appear at the bottom right of the photosphere, then approach the centre of the solar disk before curving downwards.
There is also an interesting optical effect to observe called the “blackdrop effect”. This effect acts a bit like a water drop where the tiny shadow looks like a teardrop connected to the blackness of space when it’s near the edge of the solar disk. This occurs in different places throughout astronomy. David Stewart and myself were the subject of an astronomical article written by Armagh Observatory director Mark Bailey OBE, where we both captured this effect between Saturn and its rings.
Public observing events
There will be many public events throughout the UK on Monday where you can go along and see the transit of Mercury for yourself. Check out the website GoStargazing below for locations near you.
Royal Observatory, Greenwich Live Stream
For further information, check out the Time and Date website below.
Some great information about how the orbit of Mercury proved general relativity, by Dr. Becky Smethurst.