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  #1  
Old April 11th 04, 06:23 PM
david
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Default terminology question

Just reading the latest issue of Pop. Astronomy and on page 21, in an
article about the transit of Venus, it states that the event is rar, each
transit ctually occuring in pairs eperated by over a century.

Then it says, and I quote,

"Currently, transits of venuse follow in the order 121.5, 8, 105.5, 8 years
and they will continue in this sequence for another 1000 years".

Now what the heck does that mean?


David


  #2  
Old April 11th 04, 06:37 PM
Stephen Tonkin
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david wrote:
"Currently, transits of venuse follow in the order 121.5, 8, 105.5, 8
years and they will continue in this sequence for another 1000 years".

Now what the heck does that mean?


After this one, the next will occur in 8 years time, the one after that
will occur 105.5 years later, the next 8 years later, the next 121.5 yrs
later, the next 8 yrs later, the next 105.5 yrs later..... you get the
picture?

Best,
Stephen

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  #3  
Old April 11th 04, 09:14 PM
david
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thanx.

D
"Stephen Tonkin" wrote in message
...
david wrote:
"Currently, transits of venuse follow in the order 121.5, 8, 105.5, 8
years and they will continue in this sequence for another 1000 years".

Now what the heck does that mean?


After this one, the next will occur in 8 years time, the one after that
will occur 105.5 years later, the next 8 years later, the next 121.5 yrs
later, the next 8 yrs later, the next 105.5 yrs later..... you get the
picture?

Best,
Stephen

Remove footfrommouth to reply

--
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +



  #4  
Old April 12th 04, 12:23 AM
Martin Frey
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Default

"david" wrote:

Just reading the latest issue of Pop. Astronomy and on page 21, in an
article about the transit of Venus, it states that the event is rar, each
transit ctually occuring in pairs eperated by over a century.

Then it says, and I quote,

"Currently, transits of venuse follow in the order 121.5, 8, 105.5, 8 years
and they will continue in this sequence for another 1000 years".

Now what the heck does that mean?


David


Did you spot that 8+105.5+8=121.5? The pattern isn't quite as odd as
it looks at first sight.

While the earth goes round 121.5 times, Venus goes round 197.5 times +
4 hours - so it they start together they'll not only be together
again, they'll be at exactly the same point in their orbits 121.5
years later. The 4 hour discrepancy means the pattern of transits
won't last for ever but it'll see us out and another 1000 years.

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47
  #5  
Old April 12th 04, 09:46 AM
Mike Dworetsky
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Default



"Martin Frey" wrote in message
...
"david" wrote:

Just reading the latest issue of Pop. Astronomy and on page 21, in an
article about the transit of Venus, it states that the event is rar, each
transit ctually occuring in pairs eperated by over a century.

Then it says, and I quote,

"Currently, transits of venuse follow in the order 121.5, 8, 105.5, 8

years
and they will continue in this sequence for another 1000 years".

Now what the heck does that mean?


David


Did you spot that 8+105.5+8=121.5? The pattern isn't quite as odd as
it looks at first sight.


To me it looks like 8 + 105.5 = 113.5.

While the earth goes round 121.5 times, Venus goes round 197.5 times +
4 hours - so it they start together they'll not only be together
again, they'll be at exactly the same point in their orbits 121.5
years later. The 4 hour discrepancy means the pattern of transits
won't last for ever but it'll see us out and another 1000 years.


Actually they will be at the opposite nodes, halfway around their orbits....

My take on this is that the 4 hour discrepancy means that the locations on
Earth that see the next transit in the series, but at the same node, will be
at different longitudes. A bit like solar eclipse saros.

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47



  #6  
Old April 12th 04, 03:01 PM
Martin Frey
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"Mike Dworetsky" wrote:

My take on this is that the 4 hour discrepancy means that the locations on
Earth that see the next transit in the series, but at the same node, will be
at different longitudes. A bit like solar eclipse saros.


More significant is that Venus will take a slightly different (but
parallel path across the Sun, until, after 1000 plus years it will
miss altogether: this particular Saros will come to an end.

Judging by Starry Night the June tracks are going to move up (towards
and beyond the centre of the Sun's disc) as the centuries go by, and
the December tracks move down.

In fact Starry night gives Venus close to but missing the Sun
altogether in December 2854 - as does SkyMap Pro. I shall have to
scratch that one out of my diary. Damn - I'd cancelled my skiing
holiday for it..

Presumably there will be some grazing transits at some point, transits
visible from say indide the Arctic Circle, but not elsewhere - which
would kill the parallax possibilities stone dead.

Kepler, Horrocks and Halley were fortunate in hitting just the right
part of the transit Saros.

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47
  #7  
Old April 12th 04, 08:22 PM
Yokel
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Default

"Stephen Tonkin" wrote in message
...
david wrote:
"Currently, transits of venuse follow in the order 121.5, 8, 105.5, 8
years and they will continue in this sequence for another 1000 years".

Now what the heck does that mean?


After this one, the next will occur in 8 years time, the one after that
will occur 105.5 years later, the next 8 years later, the next 121.5 yrs
later, the next 8 yrs later, the next 105.5 yrs later..... you get the
picture?


To expand on this, the reason for the 1000 year comment may be that the
transits do not always occur in pairs. Very occasionally there is just one
almost central transit, while normally there are a pair, one of which
crosses the northern side of the Sun's disk and one the southern side.
IIRC, the next time this will happen is some time in the early "3000"s,
hence this comment in the quote above.
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