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- From: chaloux@mandolin.mitre.org (Dave Chaloux)
- Newsgroups: sci.astro
- Subject: Re: Lunar astronaut covers earth with thumb
- Message-ID: <1992Nov12.223805.3577@linus.mitre.org>
- Date: 12 Nov 92 22:38:05 GMT
- References: <30473@scicom.AlphaCDC.COM> <92316.182203LABBEY@GTRI01.GATECH.EDU>
- Sender: news@linus.mitre.org (News Service)
- Reply-To: chaloux@mandolin.mitre.org (Dave Chaloux)
- Organization: The MITRE Corporation, McLean, Va
- Lines: 32
- Nntp-Posting-Host: mandolin.mitre.org
-
- Leonard Abbey writes:
-
- >It is a common illusion (optical and pshological) that the Moon is a large
- >object as viewed from the Earth. It is actually only 0.5 degrees.
-
- >At its widest point, my thumb is about 1" wide, and I can extend it roughly
- >24" from my eye. Therefore my thumb subtends an angle of about 0.04 radians,
- >or about 2.5 degrees (rough calculations).
-
- >Since the Earth is roughly three times as large (in diameter) as the Moon,
- >it would subtend an angle of 1.5 degrees as viewed from the Moon. Plenty
- >small enough to be covered by a thumb.
-
- The fact the moon is only about 30' in size has some interesting implications
- for planetary observing (especially for Mars). When I go out and look at the
- full moon I can see LOTS of detail with the naked eye. In fact, I once read
- a book that recommended looking at and sketching the moon with the naked eye
- as great practice for planetary observing. Now Mars at opposition is going to
- be about 15" in diameter. This is 120x smaller than the moon. Therefore if
- we use a telescope at 120x it subtends the same angle. At least in theory a
- three or four inch (76mm to 100mm) telescope working at 120x will show a
- lot of detail on Mars as well. In reality, I think that I personally see
- more detail on the moon at those magnifications. I think I gather from this
- that the contrast of the features on the moon is higher.
-
- Still, this little bit of calculation should give hope to those with
- relatively small scopes. This also helps explain why 5 and 6 inch refractors
- are popular as planetary scopes. You do not need a huge scope to give
- good planatary views. You do need a scope that will give absolutely crisp
- views at 120x or more. A 6" refractor working at a crisp 240x is going to
- give a wonderful view of Mars with about 4 times the angular area seen as the
- moon seen with the naked eye.
-