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Discussing observing Venus soon after sunset, Bill had written-

>>> About sunset at N 40d 28' W 86d 56' January 13,
>>> 2006 the suns declination will be approx. S22d 22' and Venus's S15d 56'.
>>> So
>>> for several minutes after sunset Venus will be above the horizon.  With
>>> a
>>> difference of 6d 26' declination, a sliver of Venus's lower limb should
>>> be
>>> illuminated .  As Venus will be approx. 1 arc minute in diameter, a 30
>>> power
>>> telescope should make it quite visible for a couple of minutes.

and I replied-

>> I doubt if Bill will have much hope of seeing Venus under those
>> circumstances. The difference in altitude between Venus and Sun will only
>> be
>> 3 degrees or so. And it would be asking for trouble to start looking
>> before
>> the Sun has completely set.

Now Bill has asked-

> We start with a approx 6.4d difference in declination, back out the sun's
> SD, and account for refraction, would the separation not be greater than
> 3d?

Even if the separation is 6.4 degrees, that is usually at a slant-angle to
the horizon when near setting.

The difference in ALTITUDES, at setting, will never be greater than that,
and usually less.

Think, if you were observing setting sky-objects from somewhere on the
equator. Then all objects, stars, planets, Sun, Moon, that have the same
hour-angle will set simultaneously, and their declination will make no
difference at all. That's an extreme case, to bring home the message.

The difference in altitudes near setting depends on the observer's latitude,
and both declinations and hour-angles. You can calculate the altitudes, near
the moment of setting, just as if you were going to use those two bodies for
a position line.

I very much doubt whether Venus could be seen at such a low altitude, when
it's so close to the Sun, and therefore in a very bright patch of the
evening sky. Itwould be a good test of Bill's observing powers, and I hope
that my words won't discourage him from trying.

George

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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