Whatever happened to Megrez?

Forums General Discussion Whatever happened to Megrez?

Viewing 14 posts - 21 through 34 (of 34 total)
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  • #582171
    Alan Thomas
    Participant

    Thanks Jeremy!

    #582172
    Chris Dole
    Participant

    Hello all.

    Just to add that a look upwards last night from my fairly light polluted location revealed Megrez to be ‘missing’. If I looked a little longer I could just about make it out. This has never been the case in the past and the familiar pattern has always been obvious.

    Chris.

    #582173
    Nick White
    Participant

    What is the reason for the scepticism about the V-band measurements? Is the scatter not similar to AAVSO visual data, albeit for a different frequency of observation? Either way, a few days of continued observation should resolve the matter, should it not?

    #582175
    Alan Thomas
    Participant

    Naked eye observation at 20.26. Megrez not visible.

    #582177
    Robin Leadbeater
    Participant

    I just had a look at the last 6 months data of a similar magnitude star with variability  that I am familiar with  and which has good photometric data in the AAVSO database.

    Here is KWS

    and here is AAVSO with the same Y scale range

    I looks like KWS is unreliable at least with bright stars in V

    Robin

    #582178
    Nick White
    Participant

    Thanks Robin, I see your point now! Cheers.

    #582179
    Alan Thomas
    Participant

    Naked eye observation 21.59. Megrez barely visible.

    #582180
    Alan Thomas
    Participant

    Naked eye observation 22.50. Megrez barely visible with averted vision. The Plough is looking a bit like Trafalgar Square without Nelson. Of course it may be something or nothing – but it doesn’t look right.

    #582181
    Chris Dole
    Participant

    Just went outside (11pm ish) and was lucky enough to catch a fair sized gap in the clouds here in Berkshire. Megrez appeared as I would expect, easily seen. It was certainly much harder to see last night. Perhaps haze? 

    Chris.

    #582182
    Alan Thomas
    Participant

    I don’t know what’s going on, but it makes a change from coronavirus!

    #582183
    Robin Leadbeater
    Participant

    Cloudy here but I grabbed a frame off an all sky camera (University of Hertfordshire   http://observatory.herts.ac.uk/allsky/ )  and compared it with the same date and time a year ago.  See attached. A crude comparison but the relative brightness of Megrez looks roughly the same to the camera at least

    Cheers

    Robin

    #582184
    Alan Thomas
    Participant

    That’s interesting, though the camera is more sensitive than the eye. I took a few shots myself and M is clearly visible.

    Here’s a (possibly dodgy) hypothesis:

    1. The current ‘lockdown’ has reduced the volume of pollutants in the atmosphere. 

    2. This has increased the effects of light pollution.

    3. This will be especially significant in urban areas (such as here, between Manchester and Liverpool).

    4. Ergo a) stars around the mag of Megrez will be pushed closer to the limits of naked-eye visibility. The effect on Megrez itself is especially noticeable because of its prominent location. b) this effect will be more visible from urban (heavily light polluted) sites than rural ones.

    In principle, these implications appear to be amenable to test fairly easily. 

    However, this reasoning seems to imply (counter-intuitively) that atmospheric pollution reduces light pollution which improves seeing!

    #582186
    Jeremy Shears
    Participant

    Certainly the transparency over the last few nights has not been 100% in this part of Cheshire. Something like a very thin veil that means the sky lacks that sparkle. this shows up in my CCD images as a high background. Even this morning the sky, though blue, lacks that deep blue.
    This reduces the naked eye limiting mag, although Megrez has always been clearly visible to me.

    But I suspect this is a meteorological situation independent of lockdown 

    #582195
    Tracie Heywood
    Participant

    I can’t comment on V light curves, but I am always wary of visual estimates of the magnitude of Megrez. The comparison stars typically used (alpha Dra, chi UMa, psi UMa, lambda Dra) are quite a distance away on the sky from Megrez and can for much of the year be at significantly different altitudes and thus affected differently by any haze present. This inevitably affects the reliability of visual estimates made under such conditions and increases the visual light curve scatter. (This issue also affects other ‘isolated’ stars such as epsilon Peg, for which there have been claims of large brightness changes over the years). It is also the case that visual magnitudes made when the star or its comparisons are close to the limit of visibility show more scatter than when the stars involved are easily seen.

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