Astronomical Close Encounters - A Matter of Perspective

There are a few astronomical events that are sure-fire crowd pleasers, guaranteed to catch the attention of astronomers and the general public alike. Who can forget the Chelyabinsk meteor, or not be blown away by a beautiful display of aurora? The problem is it’s hard to predict when and where the next sizeable lump of space rock is going to burn up in our atmosphere and aurora, though more predictable, are in general confined to latitudes more northerly than those of the majority of the UK - certainly for the spectacular displays.

Here I’m going to concentrate on equally spectacular events that are totally predictable and therefore accessible to anyone who has patience or a willingness to travel - eclipses, transits, occultations and conjunctions. Before continuing some simple definitions might prove useful:

A conjunction occurs when two astronomical bodies (two solar system objects, or one solar system object and a star) have the same right ascension when observed from Earth; this basically means that they appear close to each other in the sky. The key point here is that they appear close to one another, this is, as the title of this post suggests, merely a matter of perspective. Of course very rarely you can get conjunctions where this isn’t the case, comet C/2013 A1 Siding Spring and Mars not only appeared close in the telescope eyepiece in the second half of October 2014 but were also physically very close to one another (astronomically speaking), scarily so for any Martians out there!
If the bodies have the same declination at the time of conjunction then the one that is closer to Earth will pass in front of the other and syzygy takes place. If a smaller body passes behind an apparently larger one this is an occultation, where the smaller body passes in front of the larger one this is a transit and when the transit or occultation is between the sun and the moon then this is an eclipse, simple. Of course now we can also detect transits outside our solar system, the transit method being an incredibly useful tool for the discovery of exoplanets, but let’s confine ourselves to events closer to home for now.

Eclipses

No other astronomical event can rival a total solar eclipse for pure spectacle. It is a wonderful coincidence that the Moon is 400 times smaller than the sun but 400 times closer to the Earth (it varies hence annular eclipses which I’ll cover later). When alignment is favourable the new Moon passes directly in front of the sun from the point of view of an observer on the Earth’s surface causing the Moon to completely cover the Sun’s disc resulting in a total eclipse and the beautiful corona making up the Sun’s outer atmosphere being revealed.

I can remember travelling to Cornwall in 1999 to witness a rare total eclipse visible from mainland UK, something I’d been looking forward to for years. Unfortunately as everyone in the UK knows totality was clouded out apart from a few lucky souls who managed to find themselves under a brief gap in the clouds. Despite the huge disappointment even the cloud diluted experience was totally unforgettable, sitting on a beach watching the sky darken and the birds roosting was totally surreal and an event I’ll remember for the rest of my life. The next total eclipse visible from the UK is in 2090, one I will certainly miss but maybe one for my kids. If you can’t wait that long, and didn’t make it to the States to witness the eclipse in August 2017 then make your way to Spain in August 2026, or witness a large partial from here in the UK. If you want to find out when the next eclipses are from your location or worldwide then www.timeanddate.com is a good resource.

The closest I've come to witnessing a solar eclipse!

Of course the Sun and the Moon don’t just provide total eclipses for us to enjoy. When the moon is at a point in its orbit where its apparent size is not large enough for it to cover the entire face of the Sun (apogee) an annular eclipse occurs in which the outer edge of the Sun is still visible creating the famous ‘ring of fire’. It’s a long wait for the next one of these visible from the UK but we can console ourselves with partial eclipses and here the odds are much more favourable. I have seen a few from the UK and listed those up to 2028 below.

Partial Solar Eclipses visible from the UK 2018 - 2028 - 10th June 2020, 25th October 2022, 29th March 2025, 12th August 2026, 2nd August 2027, 26th January 2028

Of course the Moon also experiences eclipses, where the Earth passes in front of the Sun from the point of view of anyone fortunate enough to be standing on the lunar surface. These only occur on the night of a full moon and unlike solar eclipses are visible from anywhere in darkness with the moon above the horizon while the eclipse is taking place. Also as the Earth casts a much larger shadow than the moon lunar eclipses last for hours rather than the few minutes of totality experienced on Earth during a Solar eclipse.

Lunar eclipses come in three forms, total, penumbral and partial. Total eclipses occur when the Moon is completely within the darkest (central) portion of the Earth’s shadow – the umbra. A penumbral eclipse occurs when the moon passes through the outer regions of the Earth’s shadow – the penumbra. This only partially blocks the Sun, resulting in the portion of the moon in shadow becoming slightly darker and can be quite subtle. A partial lunar eclipse occurs when part of the moon passes into the Earth’s umbra.

At the point where the Moon is completely within the Earth’s umbra it takes on a reddish appearance due to the scattering of light by the Earth’s atmosphere, the same process that gives us our beautiful red sunrises and sunsets. Interestingly the more dust in the atmosphere the redder the Moon will appear. The last total lunar eclipse visible from the UK was in September 2015 when the sky was amazingly clear over London.

The Moon and the stars, a few moments before totality

Lunar eclipses visible from the UK over the next 10 years are listed below.

Total Lunar - 27/28th July 2018, 16th May 2022, 7th September 2025, 31st December 2028
Penumbral - 10th January 2020, 5th June, 5th July 2020, 25th March 2024. 20/21 February 2027, 6th Jul 2028
Partial Lunar - 16/17th July 2019, 19th Nov 2021, 28th Oct 2023, 18th Sep 2024, 14th Mar 2025, 28th Aug 2026, 12th Jan 2028

Transits

Whenever considering transits it is usually the transit of Venus that immediately springs to mind. Transits of Venus loom large in scientific history and in terms of pure adventure surely nothing can compete with the first voyage of Captain James Cook whose aims were to observe the 1769 transit of Venus from Tahiti thus enabling the accurate measurement of the distance from the Earth to the Sun, and perhaps as an afterthought, confirm the existence of the land mass we now call Australia.

Venus resembles a perfectly formed roaming sunspot as it transits, the fact that you are seeing a planet describing it’s orbit being played out before you in real time makes this an incredible event to witness. Transits of Venus are also precious for their rarity, consider the transit of 1631, if you missed that you only had 8 years to wait until the next one in 1639. Miss that however and it would be a 122 year wait until the next 8 year cycle in 1761/1769, odds then become more favourable if you miss these as there was ‘only’ a 105 year wait until the next in 1874, then another 122 year gap and so on. I’m sure many of you witnessed the transits in 2004 and 2012, unfortunately I didn’t (clouds) and fear I won’t be around for the next in 2117 regardless of the fact that it is one of the shorter gaps in the cycle!

Not to be disheartened there is also the support act of transits of Mercury. Here the odds are much better owing to the fact that Mercury orbits much closer to the sun giving it a much shorter orbital period, tis means everything is played out in fast forward. There are roughly 13 to 14 transits of Mercury per century so there is always a good chance of seeing at least one in your lifetime. These transits occur in May or November with the last one occurring on May 9th 2016, luckily the skies cleared for a few hours enabling me to capture the images below. The next three transits will occur in November 2019, 2032 and 2039 before the next May transit in 2049.

Mercury transiting the Sun - white light, false colour

If you are impatient and transits of Mercury are still too rare for you then Jupiter comes to the rescue. The four Galilean moons, Io, Europa, Ganymede and Callisto regularly transit across the face of the planet, in fact you can also witness occultations and even eclipses where one moon may move in front of another. There are a number of web resources that are incredibly useful in planning when best to view Jovian transits such as www.calsky.com, planetarium software and the astronomy press. These events make a great imaging opportunity where the shadow of the moon, or moons cast a shadow on Jupiter’s cloud tops. You can also make a time-lapse showing the Galilean moons dance around the gas giant.

Transit of Europa across the face of Jupiter

We’re not finished there however as with luck you can also witness the International Space Station (ISS) transiting the Sun or Moon. I’ve found two web resources invaluable for determining when a transit is going to be observable from my location www.calsky.com and www.heavens-above.com. I’ve covered this in detail in a separate blog post, available here http://www.thelondonastronomer.com/it-is-rocket-science/2017/6/26/imaging-and-processing-solar-and-lunar-transits-of-the-international-space-station

Occultations

If you want to observe an occultation then it’s best to aim your gaze towards Jupiter and its moons. As mentioned earlier an occultation of one of the main Galilean satellites by Jupiter is fairly common and easily observable with the equipment available to most amateur astronomers. You can plan your observing using the resources mentioned previously.

Putting Jupiter to one side, when most people think an occultation it is the Moon that plays the part of the larger body and a planet, minor planet or star that is occulted. For a star to be occulted it needs to be close to the ecliptic with the bright stars Regulus, Spica, Antares and Aldebaran being in positions where the Moon may pass in front of them. The Moon may also occult The Beehive Cluster (M44) and The Pleiades (M45).

An occultation of a planet however is what is really required to be considered a ‘crowd pleaser’. These occur surprisingly often from somewhere on the Earth’s surface, as both the Moon and the planets inhabit the area around the ecliptic and so come in to contact with one another on a regular basis. A great resource for occultation data is the USNO On-line Astronomical Almanac http://asa.usno.navy.mil/SecA/olist18.html well worth checking out to see if there is a notable event visible from your location, you can also keep an eye on the astronomy press..

Conjunctions

Finally we have conjunctions, although you might consider them the poor relation of the occultation, a celestial near miss, they can be every bit as spectacular as any of the other events I’ve described and when a conjunction is between two bright planets it can stir up a fair bit of media interest.

We also have the situation where one person’s occultation is another’s conjunction. An occultation by the Moon of Saturn for example will only visible from specific points on the Earth’s surface, but anyone near those points will have seen a conjunction of varying proximity depending their location. In the UK we’ve had some beautiful conjunctions to enjoy where the Moon has made a close pass of Venus, Mars, Saturn and even distant Uranus. As always with these events use the resources out there to make sure you don’t miss a great opportunity to witness or image a conjunction for yourself.

All the events I’ve described, while no longer offering profound scientific insight still have the power to fuel the imagination and trigger a life long interest in the heavens, so get planning and make sure to persuade your friends and family to take the time to look up and enjoy the spectacle for themselves, you never know you might make a convert.