## 13444Re: Entering seconds for custom object autostar 497

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• Aug 15, 2013
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--- In ETXASTRO@yahoogroups.com, Chris Harvey <chris@...> wrote:

> I still don't understand why Stellarium is showing equatorial
> lines spaced by 5mins; thus Pleiades looks like it is 5 arc mins
> across, yet SEDS says its 110 arc mins across .. ?
> 5 mins makes sense. What is SEDS on about exactly?
> I have Googled/Wikiepedia'd this but I'm travelling in
> a 'circle of confusion' (not good if you've had a beer).

I can see (at least) two sources of confusion:

(a) when talking about things in the sky, and looking at grid lines in a planetarium program... there are TWO DIFFERENT MEANINGS of "minutes".
One is "arcminutes"... those are based upon a 360 degree circle, which each degree in that circle being chopped into 60 arcminutes.
(the full moon is about 30 arcminutes wide, as is the sun)

The *other* meaning of "minutes" is associated with the numbering system used for the celestial equator: Hours of Right Ascension.
In this (crazy) case, there are "24 hours" defining a full circle.
Each of those hours is divided into 60 minutes (and then those go to seconds, which was the original topic of this thread).
Unfortunately (for avoiding confusion) *these* "minutes" are *fifteen times bigger* than "arcminutes".
So a constellation may span 5 RA minutes, but that therefore means that is going to be (5*15=) 75 angular arcminutes wide.

Therefore, when reading (and writing) about angles, it is very important to add a few words, such as "5 minutes of RA" or to be sure to keep saying "arc" when you mean "5 arcminutes" or "5 angular arcminutes".

Since astronomers also care about time, saying "5 minutes of time" or "on the clock" helps avoid that mud puddle.
..for a minute or so...

have fun
--dick
(b) oh, i said "at least two" didn't i? The Pleiades doesn't have distinct "edges", and there are associated nebulosities.
So one measurement may have been talking about the 6-star asterism, and the other may have been adding in a lot of the surrounding fluff.
Example: http://apod.nasa.gov/apod/ap981025.html
I'm sure SEDS is including the fluff:
http://messier.seds.org/m/m045.html
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