This answer includes a stunning photo of the Moon and Jupiter "Trails of the Shifting Moon" by photographer Jin Lu.

The caption reads:

From the photographer: "I spent 4 days capturing the trail of the moon from the balcony of my house. This is a composite of 90 pictures, showing the shift of the path of the waxing Moon from crescent phase to near first quarter. Random floating clouds decorated the trails. The interval between each day's sequence of pictures ranges from 1.5 to 2 min. Besides the Moon, the most obvious celestial object in the composite is Jupiter. The trail of Jupiter is from pictures of Jan 9, the day the moon appeared closest to Jupiter in the evening sky. Although Jupiter looks closer to the Moon on Jan 8, that is not how it actually appeared then." - Jin Lu

I'm confused by two things:

  1. The Moon is shown for January 7, 8, 9, 10 and the caption says "The trail of Jupiter is from pictures of Jan 9". I'd expect Jupiter to have contributions from all four days, not just one day. How can that be?

  2. "Although Jupiter looks closer to the Moon on Jan 8, that is not how it actually appeared." If I measure the distance in pixels from the top-left Jupiter to the top-left Jan 8 and 9 Moons, the Jan 8 Moon is closer. How could that not be true in reality then?

enter image description here


1 Answer 1


Looking toward the western horizon from 33°N latitude, hour angle increases in the 11-to-5 o'clock direction, and declination increases in the 8-to-2 o'clock direction. The Moon was at -2.8° declination on Jan 8 and +2.2° on Jan 9, while Jupiter was near -2.0° on both nights. Where they overlap in hour angle, the Jan 9 Jupiter images appear closer to the Jan 8 Moon images.

For the Moon to appear at similar hour angles on consecutive nights, the photographer must have started taking images ~40 minutes later each night, increasing Jupiter's hour angle ~10° per night. On Jan 7 and 8, Jupiter was probably still above the ~20° high frame at the ends of those short series of exposures. When the Jan 10 series began, Jupiter was likely already behind the building.

The scenario is reproducible in Stellarium if the location is in the southwestern US and times are evenings in January 2011.

  • $\begingroup$ Your answer is much better than mine. $\endgroup$
    – Rory Alsop
    Mar 17, 2019 at 20:04
  • $\begingroup$ Nicely done, thank you! I don't know why, but I could not get out of the mindset that the same set of times were used each night. $\endgroup$
    – uhoh
    Mar 17, 2019 at 23:01

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