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Observing and Imaging Group
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Topic starter
Just received the BAA handbook for 2016, and there is a nice 2 page feature on one of the astronomical highlights of next year - the transit of Mercury on 9 May 2016.
The transit will be visible in its entirety from the UK. While not as spectacular as a transit of Venus (only another 112 years to wait until the next one of these!) it's still a reasonably rare event (about 13 or 14 per century) and will definitely be worth viewing.
I'm sure we will be running a major event at the Royal Observatory Greenwich to view it!
The last transit of Mercury was in 2006 and was not visible from the UK as the Sun had set. The previous transit in May 2003 was visible, but the Sun was very low at the start of the eclipse and everything had finished by 11.30am.
It's actually difficult to find a transit that is as long and as favourable to observers in the UK as the one coming up in May next year... perhaps the one in 1832... but in many cases, either the start or end of the transit is missed, or the Sun is very low, or the transit is very short. See here.
So on Monday 9 May, 2016, these are the timings of the eclipse from Greenwich:
- Exterior ingress: 11:12:23 UTC (12:12:23 BST)
- Interior ingress: 11:15:35 UTC (12:15:35 BST)
- Maximum: 14:56:17 UTC (15:56:17 BST)
- Interior egress: 18:37:21 UTC (19:37:21 BST)
- Exterior egress: 18:40:33 UTC (19:40:33 BST)
The duration of the transit is therefore 7 hours 28 minutes 10 seconds!
The Sun's altitude at the start of the eclipse is nearly 55 degrees. At maximum, it is 41 degrees high, and at the end of the eclipse 7 degrees. Azimuth ranges from 161 degrees in the south to 289 degrees in the west.
So how big will Mercury be? In the last transit of Venus, Venus was 57.8 arcseconds across, or 0.03 times the diameter of the Sun. Mercury will be just 12 arcseconds across, or just 0.006 times the diameter of the Sun - just 20% the size of Venus!
To put this into further context, research has shown that a sunspot must have a penumbral diameter of about 41 arcseconds and an umbral diameter of about 15 arcseconds to be visible to the average naked eye. This means (I think) that the transit of Mercury won't be visible without a telescope... or, at least, it will be very difficult to see with the naked eye.
Through a telescope, it will look pretty good - about the size of a medium-sized sunspot - gradually moving across the disc of the Sun.
Posted : 01/10/2015 12:48 pm
Sorry Mike,
Can you explain :- "The Sun’s altitude at the start of the eclipse is nearly 55 degrees. At maximum, it is 41 degrees high, and at the end of the eclipse 7 degrees."
Start 55 deg. Maximum 41 deg.?
Posted : 02/10/2015 2:29 pm
Topic starter
Hi Bill,
By "maximum", I mean the point of maximum or greatest transit (the moment when Mercury passes closest to the centre of the Sun).
Hope this explains it better.
[EDIT: This point is also known as the point of "minimum separation", just to confuse things more!
Minimum angular separation is measured in ‘arc-seconds’ between the centre of Mercury and the centre of the Sun and it occurs at the instant of maximum or greatest transit.]
Posted : 02/10/2015 3:08 pm
Thanks Mike,
I was confused and I couldn't see you making a silly error.
I can because, I'm a new member and an amateur.
Posted : 02/10/2015 8:20 pm
It just occurred to me: I've never seen Mercury. I've had a couple of attempts at looking for it but it's never been sufficiently above the setting or rising sun to see it without going blind.
So it just occurred to me how ironic it would be if the first time I 'saw' Mercury was during the transit, when you can't actually see the planet itself at all, just the disc-shape created as it blocks a tiny portion of the Sun's light.
Posted : 03/10/2015 12:32 am
Topic starter
It just occurred to me: I’ve never seen Mercury
I've spoken to many, quite experienced, observers who've never seen it, so you're not alone!
To be honest, you may well have seen it, but didn't recognise it as a planet. It is, of course, always pretty low down on the horizon, so looks like a small red star most of the time.
Through a telescope... I know I shouldn't say this... but it's often underwhelming. Yes, you can often see Mercury as a beautiful phase (when it's a crescent, it's particularly nice), but its position low on the horizon means you are looking through a lot of air mass. As a result, the image is often wobbly and ill-defined.
Still, I always make a point of trying to view it when I can.
One occasion was particularly scary, though! I was driving down to Gatwick at about 5.30am one morning, when I saw both Venus and Mercury in the sky on the left hand side of the car, as I hammered down the M23. "Oh wow", I thought, "that's really beautiful"... fortunately, I had enough wits about me to look forward again, as a large queue of traffic suddenly came out of the blue.
That could have been the first motorway pile-up caused by astronomy! Try explaining that one to the police!! Disaster was averted, fortunately.
Posted : 03/10/2015 9:07 am
"Well, you see, officer, there were these two objects in the sky, and..."
"Breath in here please, sir. Nice long breathe..."
Posted : 03/10/2015 9:20 am
Thanks Mike,
I was confused and I couldn’t see you making a silly error.
I can because, I’m a new member and an amateur.
Bill, do you have Stellarium installed? I thought I video capture a Stellarium simulation of the Mercury transit to accompany Mike's stats: (you need to maximise the window and change the resolution to 480p by clicking the cog wheel in bottom right corner). I have no idea how to upload in higher resolution 🙁
(Edit: Figured out how to do it in HD, so upload replaced)
Sorry its not hi res but you can still see Mercury arching from middle left limb to bottom limb. It looks very tiny! I assume Stellarium is showing that transit to scale because I compared to the Venus transit and that transit looks exactly to scale compared to the images I have seen.
I think white light filter is the order of the day as Mercury might look quite lost in contrast on a H-Alpha scope!
Posted : 03/10/2015 3:57 pm
Wow. Cool. Interesting inverse 'J' shape in Mercury's transit. I wasn't expecting that.
Posted : 03/10/2015 4:06 pm
Topic starter
I think white light filter is the order of the day as Mercury might look quite lost in contrast on a H-Alpha scope!
Yep, agree with that. White light is definitely the way to go... but some H-alpha images may also be worth trying, particularly if you can get Mercury going "through" a prominence feature. In white light, let's hope there's a few sunspots nearby, as that would make some images very special.
Wow. Cool. Interesting inverse ‘J’ shape in Mercury’s transit. I wasn’t expecting that.
Indeed. Just a consequence of the transit being 7 hours long. It's the rotation of the Earth that's causing most of that shape, though Mercury also has an apparent motion that's adding to the complexity.
Posted : 03/10/2015 4:45 pm
