Astronomical Close Encounters: A Matter of Perspective
- Created: Sunday, 12 October 2014 14:55
- Written by Roger Hutchinson
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 meteor burning up in the skies over Chelyabinsk, Russia in February 2013? Or not be blown away by beautiful images of aurora such as this years winning entry in the Astronomy Photographer of the Year competition?
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, apart from the odd rare occasion, a phenomena 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 is when two astronomical bodies (two solar system objects, or one solar system object an 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 article 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 will not only appear close within the telescope eyepiece in the second half of October 2014 but will also be physically very close to one another, 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 are able to detect transits outside our solar system, the transit method being hugely successful in the discovery of exo-planets, but let’s confine ourselves to events closer to home for now.
No other astronomical event can rival a total solar eclipse for pure spectacle. It is a crazy coincidence that the Moon is 400 times smaller than the sun but also at times exactly 400 times closer to the Earth (it varies hence annular eclipses which I’ll cover later). When alignment is favourable the new Moon can pass directly in front of the sun from the point of view of the 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 view a rare total eclipse visible from mainland UK, one I’d been desperate to see since reading as a child about the 1927 Giggleswick eclipse in one of Patrick Moore’s excellent popular astronomy books. He mentioned that the next eclipse would be in 1999 which at the time of reading seemed impossibly far away but I made it a date and kept my side of the bargain. Unfortunately as everyone in the UK knows the weather didn’t and totality was clouded out apart from a few lucky souls who managed to find themselves under a brief gap in the clouds. Even the cloud diluted experience was totally unforgettable though, 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 make your way to the Faroe Islands in March next year, otherwise 2017 provides a great opportunity to witness totality from mainland United States.
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 whereby 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 and the UK can enjoy five between now and the main event in 2090 with a large (80%) partial eclipse in March 2015. These in themselves are spectacular and provide fabulous imaging opportunities if you are that way inclined, so fingers crossed for clear skies next March.
Of course eclipses aren’t just to be enjoyed by us on Earth, the Moon also experiences them 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 does when it returns the favour during a solar eclipse lunar eclipses last a much longer than the few minutes of totality experienced on Earth.
Lunar eclipses come in three forms total, penumbral and partial. Total eclipses are described above where the Moon is totally 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, which only partially blocks the Sun. This results 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 so fingers crossed for a (remote and safe) volcanic eruption before the next lunar eclipse! There have been two total lunar eclipses this year, neither of which visible from the UK but well covered in the media. The table below gives the dates of those that will be visible from the UK over the next 10 years.
Lunar Eclipses visible from the UK 2014 - 2024
28th Sep 2015, 27/28th Jul 2018, 16th May 2022
16th Sep 2016, 10/11th Feb 2017, 7th Aug 2017, 10th Jan 2020, 5th Jul 2020, 25th Mar 2024
7th Aug 2017, 16/17th Jul 2019, 19th Nov 2021, 28th Oct 2023, 18th Sep 2024
Whenever considering transits you can’t help but think about the transit of Venus. 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.
While not necessarily spectacular, Venus resembles a perfectly formed roaming sunspot as it transits, it is the fact that you are seeing a planet describing it’s orbit being played out before you in real time that makes this event grip the imagination so strongly. 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 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, everything is basically 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 next one on May 9th 2016, followed by three November transits in 2019, 2032 and 2039 before the next May transit in 2049.
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 watch 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, as well as the invaluable sky guide on this website and broadcast via the Awesome Astronomy podcast. These events make a great imaging opportunity either showing the shadow of the moon, or moons on Jupiter’s cloud tops, or even better a time-lapse of a moon disappearing behind the limb of the planet.
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. It is really important you enter your exact location as these transits tend to occur along a very narrow line, once you’ve done that you can check to see if the ISS is going to do you a favour up to a few months ahead and then plan to either try to observe the event or image it. Make sure you keep checking up to the day the transit is predicted to occur as the degree of uncertainty does increase with time. I was lucky enough to catch the ISS passing in front of the sun from my London back garden last May. I never thought watching a small dot race across the face of the sun could be so exciting! You can take a look of my video of this event here.
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. Here's an annimation that I took of an occultation of the moon Callisto by Jupiterccultation of the moon Callisto by Jupiter. You can plan your observing using the resources mentioned previously.
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 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 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. The Moon has occulted Saturn from somewhere on Earths surface 11 times this year alone and on the 29th September occulted Saturn, Ceres and Vesta all on the same day! On the 25th October an occultation of Saturn by the Moon is visible from the UK but not at a particularly favorable time with Saturn’s disappearance behind the Moon beginning in daylight and finishing when the Moon and Saturn are close to the horizon. The Moon has also occulted Venus, Mars, Mercury and Uranus this year. A great resource for occultation data is the USNO On-line Astronomical Almanac, well worth checking out to see if there is a notable event visible from your location.
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. When a conjunction is between two bright planets it can stir up a fair bit of media interest, such as that which occurred between Jupiter and Venus in the pre-dawn sky of August this year, the two worlds being only about 15’ apart (about half the width of a full moon).
We also have the situation where person’s occultation is another’s conjunction. All those occultations by the Moon of Saturn for example were 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, can still lead to astonishing discoveries such as the rings around the 250 km diameter asteroid Chariklo identified as it occulted a star. They 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.