# Mercury to transit Sun: Viewer discretion advised

Mercury will transit the Sun next Monday, and those wishing to witness this occasional event without risking blindness will be pleased to know that NASA will have full coverage. The planet will creep across the Sun's fiery surface between 11:12 GMT (7:12 AM EDT, 12:12 BST) and 18:42 GMT (2:42 PM EDT; 19:42 BST). The entire …

1. #### Can someone explain . . .

why, if Mercury whizzes around the Sun every 88 days, and Earth and Mercury both orbit in the same plane, and tiny, tiny Mercury is really, really close to really big the Sun that a transit only happens 13 times a century rather than, say, four times a year?

Thank you.

1. #### Re: Can someone explain . . .

We're not quite coplanar.

There is a 7 degree difference:

Wikipedia

1. #### Re: Can someone explain . . .

But don't the relative sizes and positions allow for quite a lot of non-coplanarity and still let you see Mercury doing its dash?

1. #### Re: Can someone explain . . .

. . . or does a 'transit' demand it cross at least, say, 90% of the transited object?

1. #### Re: Can someone explain . . .

The magic of geometry . . a quick scribble reveals that little 7 degrees can throw Mercury up to 11 times the Sun's radius above/below our orbital plane. Apparently Mercury's orbit isn't as really, really close as I thought.

1. #### Re: Can someone explain . . .

The magic of geometry

And some even simpler linear algebra. It's like* if you have two buses that serve the same bus stop, one that arrives every 40 minutes, another every 45 minutes. The time between instances where both buses arrive at once is the least common multiple of the two times, which in this case would be 360 minutes. Accounting for the wobble is like saying that you only visit the bus stop every, say, 50 minutes so you're only interested in times that you're actually there. Again, you use the LCM. The LCM of 360 and 50 (or of 40, 45 and 50, if you want to combine all three values at once) is 1800, so the time between the coincidences is 1800 minutes or 30 hours.

* Obviously, this is a simplification. The bus stops would be moving, for one thing, since we're interested in colinearity rather than when planets are at fixed points. In two or three dimensions with elliptical orbits, the calculations are a bit more involved, but the basic ideas of periodicity still hold (as far as I know; please correct me if I'm wrong). The reason I'm talking about the simpler case is that it helps to understand that the LCM is fundamental to combining periods. Most notably, if the periods being combined are relatively prime, then the combined period is the product of each of the individual periods, which might be a surprising result if you didn't know about the LCM.

Incidentally, they reckon that cicadas are so successful because the period of their life cycle is relatively prime to that of the predators that keep them under control. This means that they get the maximal period between "busts" in their predator-prey cycle.

2. #### Re: Can someone explain . . .

"But don't the relative sizes and positions allow for quite a lot of non-coplanarity and still let you see Mercury doing its dash?"

Mercury isn't as close to the sun as you might believe. It's ~50km million, whereas earth is 150. So it's 7 degree incline puts it 6km million above the ecliptic (right angle triangle, 50m km base, 7 degree angle = 6m km height). At the closest approach, that can be 3.4 degrees from our perspective (100mkm base, 6mkm height = 3.4 deg).

For comparison, the sun only appears as an arc of 0.3 degrees above centre from our perspective..

2. #### Yay

Will get telescope and tripod out, and a suitable screen to project onto, as, for once, it's predicted to be clear for this.

I may, carefully tryout my sunfilter I got for an eclipse a few years back...but only in live view, not through the viewfinder

1. This post has been deleted by its author

2. #### Re: Yay

Bugger - I've just realised I've missed a golden opportunity to do a "Sic transit gloria mundi" pun - a glorious transit next Monday...

Damn - why do I always think of ideal combacks too late.

3. #### Re: Yay

I find sun viewing best with a camera on live view anyway, but I am perfectly happy to look through the eyepiece to look at the sun when I am trying to track a sunspot with high magnification, I test my sun filter every time I use it so I am not worried.

3. #### It's dangerous?

Yes you do need a small telescope to see this event and at least 60x magnification therefore:

Yes it's potentially dangerous.

Yes you could potentially be blinded.

Yes you could be knocked down by a bus tomorrow.

But no, none of this 'dangerous stuff' will happen with a bit of care and common sense. Safe solar viewing info is all over the web and both white light solar filters and the *correct* filter medium to make diy filters are available quite widely. No need for anything expensive - £20 will get you enough "Baader Solar Film 5.0" and cornflake packets to make several filters and you can use the filter all summer ...

Watching stuff on a screen is never as interesting as seeing it happening in front of you.

If you're worried then NASA is a great option but it need not be dangerous to watch it live yourself.

Be sensible, be safe, enjoy it.

1. #### Re: It's dangerous?

Also remember, trying to watch it from below the inevitable 10 thousand feet cloud cover from the uk may hinder your enjoyment, you may also crick your neck trying to find the sun (albeit your eyes will work another day)

4. Does this mean we'll be able to witness light bending?

1. Probably not. Mercury's measly mass would only deflect the sun's light by 0.00000000000003 of a degree.

5. Regarding eye safety: Grab an arc welding helmet and you're good to go. Note acetylene welding goggles aren't considered safe, happy viewing!

6. #### Telescope solar filters

DO NOT use the solar eyepieces that come with some cheap import telescopes. These are very dangerous

As mentioned before use Baader Solar Film at the entrance to the main aperture so filtering the light before it enters the telescope optics.

7. #### In case you were curious

Here's what can happen if you don't take proper measures when using a telescope to view the sun.

1. #### Re: In case you were curious

Feh, in a pig's eye!

8. #### Transit

So how long would it take a white-van man to take his Transit?

9. #### The next chance to catch the phenomenon live will be on 11 November 2019....

... though I gather the next opportunity to see the transit in it's entirety (from the UK) isn't until 7th May 2049; Fingers crossed for 9th May 2016 then!

10. So, should I endevour to see this from Mercury Bay, or will there not be enough resolution?

## POST COMMENT House rules

Not a member of The Register? Create a new account here.