Dodgy astronomy on TV - for amusement only

Sayf Connary wrote in
:

Although that would make remembering birthdays easier cause nobody
would have one! :-)

I'd have one

mike

mike ring wrote:
Sayf Connary wrote in
:


Although that would make remembering birthdays easier cause nobody
would have one! :-)


I'd have one

mike

Sorry I didn't make myself clear: Nobody would be able to celebrate one.
Sheesh, I thought people had a sense of humour around here. Guess not.

--
~Sayf

Sayf Connary wrote:
Sheesh, I thought people had a sense of humour around here. Guess not.

Aww, don't be so harsh on yourself just because Mike's humour went way
over your head.

Best,
Stephen

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JG wrote:
I might be naive but as I understand it :

1 a DAY is the time it takes for a body to revolve about its own
axis.

Correct -- but you do need to specify your reference for revolution. A
sidereal day is not the same as a solar day.

In the case of the Earth this is 24hours (all figures rounded)

Yes, for a *mean* solar day. A sidereal day is a tad shorter.

With the Moon this is the same

No it isn't. The Moon rotates on its axis once every month (again,
rotation reference is required).


2 a YEAR is the time it takes a body to revolve around its prime
focus.

Does "prime focus" have a meaning in astronomy other than its optical
one?

Earth's prime focus = Sun Year = 365days (ish)
Moon's prime focus = Earth Year = 24 hours (ish)

Even if you are redefining a year with respect to the Moon, this is
still wrong (see above).


Best,
Stephen

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JG wrote:
Jonathan Silverlight
sai d

Mark Ayliffe writes
Mike Dworetsky illuminated us
with:
"Mark Ayliffe" wrote in message
John Brockbank illuminated us with:

On West Wing last night, CJ (Allison Janney) said that the moon
day length was the same as the year length.

OK, I'll bite, what is the difference between the day length and
year length on the moon?

Incredible. What were the writers thinking of? A day on the Moon
is the same as a synodic month. A year on the Moon is the same as
a year
on Earth.

Hmm, OK I've dug around a bit and I think the best answer is that
the moon doesn't have a year. Days and months make sense, but not
years. It's confused because we have the concept _on earth_ of a
lunar year which isn't quite the same as a sidereal year. Ho hum.
Otherwise it might make sense to think of a year as being the time
to rotate around the primary focus and CJ would have been correct.
But no-one thinks like that... ;-)

All the definitions I've found say that a year is the time taken to
revolve around the sun, so I think they are right not to.

I have been lurking here for some time simply to learn but this thread
does seem to be going out of it's way to get itself lost.

I might be naive but as I understand it :

1 a DAY is the time it takes for a body to revolve about its own
axis. In the case of the Earth this is 24hours (all figures
rounded) With the Moon this is the same since it always
presents the same face to the Earth.

Relative to what? I think a day is the time it takes a body to rotate about
its own axis, but a proper reference must be chosen, there are several
possibilites. A day on earth is 24 hours relative to the Sun, but 23 h 56 m
04s relative to the stars (sidereal day). Since you define a lunar day to
be the takes for a body to revolve (rotate) about its own axis, and the Moon
always presents the same face towards the earth (on average, that is) , it
means the lunar day length must be similar to the orbital period, which is
certainly much longer than 24 hours. The orbital period of the Moon around
the Earth, relative to the stars, is 27.3 earth days. It takes 29.5 earth
days for a lunar phase to repeat itself. I.e. if you were standing on the
Moon, it would take 29.5 earth days between two sunsets. The natural lunar
day length is then 29.5 earth days, isn't it?

2 a YEAR is the time it takes a body to revolve around its prime
focus. Earth's prime focus = Sun Year = 365days (ish)
Moon's prime focus = Earth Year = 24 hours (ish)

Not quite. Which reference system would you choose for measuring the
revolution of the Moon around the Earth? The stars? Or the Sun? If you
choose the stars, the lunar year is 27.3 earth days, somewhat shorter than a
lunar day. If you choose the Sun, they are the same, 29.5 days.

When I started to write this post I had intended to say that the moon
had no Day but fortunately pure logic kicked in

:-)

Clear skies
Carsten A. Arnholm
http://arnholm.org/
N59.776 E10.457

Sayf Connary wrote:
mike ring wrote:

Sayf Connary wrote in
:

Although that would make remembering birthdays easier cause nobody
would have one! :-)


I'd have one

mike


Sorry I didn't make myself clear: Nobody would be able to celebrate one.
Sheesh, I thought people had a sense of humour around here. Guess not.


Never mind. I get it now.

--
~Sayf

In article ,
Charles Gilman wrote:
the latter is academic). The time that the Sun takes to orbit the centre of
the galaxy I call a "sunth". Any other takers for this coinage?

Do it. At least then something will rhyme with "month", which will defeat
the smartarses.

Obviously, something other than "n+1th", I mean.

Oops.
--
--------------------------------------+------------------------------------
Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/

Stephen Tonkin said
JG wrote:
I might be naive but as I understand it :

1 a DAY is the time it takes for a body to revolve about its own
axis.

Correct -- but you do need to specify your reference for revolution. A
sidereal day is not the same as a solar day.

By 'reference for revolution' do you mean :-

From when 'London' directly faces the Sun to the next time that this
occurs?

In the case of the Earth this is 24hours (all figures rounded)

Yes, for a *mean* solar day. A sidereal day is a tad shorter.

With the Moon this is the same

No it isn't. The Moon rotates on its axis once every month (again,
rotation reference is required).

Duh ! - a 'Homer' moment - of course it's a month (ish) 28.531
Earth Days ?


2 a YEAR is the time it takes a body to revolve around its prime
focus.

Does "prime focus" have a meaning in astronomy other than its optical
one?

I was simply following a previous posters reference - I take it to mean
"the body around which it rotates".

Earth's prime focus = Sun Year = 365days (ish)
Moon's prime focus = Earth Year = 24 hours (ish)

Even if you are redefining a year with respect to the Moon, this is
still wrong (see above).

Is this better :-

Moon's prime focus = Earth Moon Year = 28.531 Earth days.

JG

"Carsten A. Arnholm" said
JG wrote:

I have been lurking here for some time simply to learn but this thread
does seem to be going out of it's way to get itself lost.

I might be naive but as I understand it :

1 a DAY is the time it takes for a body to revolve about its own
axis. In the case of the Earth this is 24hours (all figures
rounded) With the Moon this is the same since it always
presents the same face to the Earth.

Relative to what? I think a day is the time it takes a body to rotate
about
its own axis, but a proper reference must be chosen, there are several
possibilites.

My intention was to simplify the discussion by avoiding the detailed
difference between 'Solar' and 'Siderial' (of which I am aware) but it
seems I have clouded it (no pun intended) even further

I hope my other reply to Stephen's post clears matters a little better.

JG

On Sun, 13 Feb 2005 19:45:22 GMT, JG
wrote:

snip
I have been lurking here for some time simply to learn but this thread
does seem to be going out of it's way to get itself lost.

I might be naive but as I understand it :

1 a DAY is the time it takes for a body to revolve about its own axis.
In the case of the Earth this is 24hours (all figures rounded)
With the Moon this is the same since it always presents the same
face to the Earth.

If you argue that it is the time interval between successive transits
of the primary, then 24 hours is close for Earth but for the Moon you
are stuck with zero or infinity since the Earth doesn't move in the
lunar sky.

If you argue that it is the time between successive sunrises (or high
noons, or sunsets, or ...), then you get the same 24 hours for Earth
but 27+ days for the Moon.


2 a YEAR is the time it takes a body to revolve around its prime focus.
Earth's prime focus = Sun Year = 365days (ish)
Moon's prime focus = Earth Year = 24 hours (ish)

The moon moves a little more than 12 degrees in 24 hours. Depending
on whether you define a complete revolution of the Moon relative to
the Sun or the background stars, you get 29+ or 27+ days,
respectively.


When I started to write this post I had intended to say that the moon
had no Day but fortunately pure logic kicked in

JG



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