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    Re: Jupiter in daylight and micrometer test
    From: Peter Monta
    Date: 2016 May 2, 00:43 -0700
    Saw Jupiter during daylight today (sun 4 degrees up).  First time ever.

    I took along my trusty Davis Mark 3 sextant, thinking to dial in the position of a lamppost using some horizon landmarks, but ended up not using it at all.  Quicker is this scheme:

    - find a tall stable object with plenty of ground in front looking toward the azimuth of interest
    - use an ephemeris app to find the azimuth and elevation at a convenient time (I used Stellarium on iOS)
    - in advance, place a marker on the ground in front of the lamppost, then walk along the line connecting marker and lamppost, watching a smartphone app showing the GPS bearing (I used SensorMonitor on iOS)
    - move the marker until the marker-lamppost line is good to within a degree or so
    - walking this line, use the phone's accelerometer/angle sensor to place the top of the lamppost at the required elevation (I used Theodolite on iOS)
    - move the marker to this golden spot
    - wait
    - acquire Jupiter in the binoculars field of view

    With the Sun 15 degrees up, Jupiter was visible in the binoculars, but it took several seconds to find, as contrast was low.  Next:

    - adjust your position so that Jupiter is slightly above some reference mark on the lamppost
    - remove binoculars, try with naked eye

    I don't know if some people can see Jupiter with the Sun at >= 15 degrees (near sea level, clear weather), but for me it was quite hopeless.  I kept trying until the Sun was 4 degrees up (by which time the binocular image looked like a searchlight, instantly visible)---finally success, and hey, this counts as daytime.

    I didn't quite have time to see whether Jupiter became harder to see if it was moved substantially away from the stable reference point.  Next time perhaps I'll move it some reasonable distance, then use a paper tube to suddenly mask out the lamppost.  One thing I did notice is that it helps to rock side to side very slightly, moving the eyes maybe an inch or so.  This generates enough parallax to cause the target to move slightly with respect to the lamppost tip, making it a little easier to see.

    The books say to prioritize twilight sights by brightness.  That's fine, but with a 7x35 sextant scope, there's no need to wait for twilight.  Venus, Jupiter, Mars at opposition, and maybe Sirius should all be good well before sunset.  The more numerous mag_v~=0 sources I don't know, but it seems worth trying a little in advance of sunset.

    Cheers,
    Peter

    ps: I suppose one of the SkyScout-like AR apps would have been another possibility, but I don't trust the phone's compass.  Either the SkyScout has a very good compass or it allows you to touch it up with a solar azimuth or landmark azimuth.

       
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