Why Do Stars Appear to Move in the Night Sky?

Question: Why do the stars in the sky appear to orbit? – Ariana

Answer: I think that you are asking why stars appear to move through the night sky from east to west in tracks that appear to be centered on the North Star.  These apparent star tracks are in fact not due to the stars moving, but to the rotational motion of the Earth.  As the Earth rotates with an axis that is pointed in the direction of the North Star, stars appear to move from east to west in the sky.

Jeff Mangum

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Identifying Strange Sounds Received by a Radio

Question: I received a radio wave with my homemade antenna that it sounds like a motorbike, like a Harley Davidson. Which type of signal it could be? it could be about the sun or the earth magnetosphere? It’s for a school presentation. Thanks.  – Gerard

Answer: The source of a radio signal depends upon the frequency that you were tuned to (among other things), but I suspect that if you were using a standard radio frequency (a few 10s of mega-Hertz) that what you received was a man-made transmission of some kind.  There are a number of places on the web which can help you identify man-made radio broadcast noises.  For example, W2SJW’s collection of strange radio noises contains numerous samples of man-made radio noises.  You might try listening to some of these sound samples to see if they sound like what you are hearing.

If you think that these might be sounds produced by ionospheric effects, you might try the Earth Sounds web site (although this web site appears to have fallen into disrepair…).  There are only three sound samples listed, and none of them sounded like a “motorbike”.

Jeff Mangum

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Star and Planet Sky Location Information

Question: Could you generate an image of the planetary alignment on September 26th,  2014?  – Branden

Answer: I think that you are probably looking for night sky planet positions for a given date.  One of the best sources for this information is the Sky and Telescope This Week’s Sky at a Glance.  Each week Sky and Telescope produces a nice summary of bright star and planet alignments that can guide you as to where to look for these objects in the sky.

Jeff Mangum

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Why isn’t the Precession of the Lunar Nodes Uniform with Time?

Question: I am an amateur naked eye astronomer.  I teach astronomy to 6th graders, I read a lot on the subject, and I do a lot of experiments and observations to better understand the movements of the sun, moon, etc.

I have been looking at the precession/regression of the lunar nodes and I am a bit confused.  I know the nodes are just imaginary lines where the titled orbit of the moon crosses the ecliptic and that they move westward, opposite the direction the earth spins and the moon orbits the earth and the earth orbits the sun and that this takes 18.6 years.  I have found vague references to the fact that this is not a steady movement but just moves in that direction on the average to complete one rotation relative to the fixed stars in 18.6 years.  I have made some simple models to test this with ephemeris listings of moon phases and when it is in the ascending or descending node, and if checked over long periods of time it does indeed seem to be working its way westward, but if checked over the short term, the position of the moon in the nodes seems to jump back and forth by quite a bit relative to the fixed stars.  So, my question is (sorry for the long explanation) what is causing this?  Is it just the complexities of the moon’s orbit that make it swing around a bit erratically each time around while adding up to a general westward precession, is it something to do with the elliptical shape of the orbit along the line of apsides between perigee and apogee that is also precessing (which I guess is a further deformation of the orbit since its orientation moves about in the opposite direction in a different amount of time: 8.85 years), or am I looking at this all wrong?  I’ve tried my best to figure this out but I just can’t wrap my head around it.  Please help!  – Tyrel

Answer: This is a rather complicated problem as it involves some rather complex interactions amongst the Earth, Moon, and Sun.  It involves the construction of a complete theory of lunar motion, which has been tackled by some of the most noteworthy physicists and mathematicians of all time, including Newton and Euler.  After consulting Orbital Motion by A. E. Roy, I think that the major contributor to this oscillation of the Moon’s position in the nodes is due to the Sun’s gravitational pull and its affect on the parallactic inequality  This effect produces a variation in the longitude of the Moon with amplitude (E-M)/(E+M)*(a/a1), where E and M are the masses of the Earth and Moon, respectively, and a and a1 are the mean geocentric distances of the Moon and Sun, respectively.  The amplitude of this term is about 2 arcminutes and has an amplitude of one synodic month.  There are other contributors to these motions that are very well described in Roy, Chapter 10.2, if you are interested in digging into this subject.

Jeff Mangum

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Strange Image of Jupiter

Question:  I have a photo that looks like a comet impact on Jupiter! I took the photos on January 22, 2014 sometime between 2:25am and 2:45am EST. My best guess would be 2:33am EST.  I posted my photos to YouTube. Please take a look at this photo slideshow, maybe you can tell what this is that I captured in my telescope!  Thank You!!  – Phil

Answer:  The strange structure you see around Jupiter is very likely due to reflections in the optics of your camera and/or saturation of the CCD detector in your camera (assuming you were using a digital camera).  Cameras quite often produce odd images of bright objects in the sky like this.  For examples, see the Unexplained Celestial Observations section of this blog.

Jeff Mangum

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Bright Spots Near the Sun?

Question:  I took several pictures of a strange sight. It is what looked like a small sun beside the sun at sunset. I will try to post a picture. Any ideas?  It was taken at sunset, July 12,2014.  – Kathy

Answer:  I think it very likely, based on your description, that what you were seeing was a “sun dog”.  Sun dogs are due to an atmospheric effect caused by the refraction of light through ice crystals that produce bright spots at an angle of 22 degrees on both sides of the Sun (though both are not always visible).

Jeff Mangum

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Why Do Stars Disappear?

Question:  Around 10:39 pm June 17th 2014 I was watching Arcturus and it disappeared for about a minute or so and then reappeared. I am asking why this happens? I have never seen it before or even heard of it happening.  – Hilligus

Answer:  In the vast majority of cases when stars are observed to “disappear”, the culprit is our own Earth’s atmosphere.  Depending upon how “thick” the atmosphere is between an observer and the star, one can find that the star can simply “dance around” when the atmosphere is not very thick, to just plain disappearing when the atmosphere is quite thick (i.e. cloudy).

Jeff Mangum


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Star Gazer Lives On

Hi Folks,

Through a very interesting article in the latest edition of Astronomy Beat, produced by my colleagues at the Astronomical Society of the Pacific (ASP), I was re-introduced to a wonderful little PBS-produced television show called “Star Gazers”.  These 5-ish minute episodes describing celestial events viewable by backyard astronomers were hosted by a guy named Jack Horkheimer from 1976 through 2010.  Jack obtained a kind of cult following for these Star Gazer (actually, it was originally titled “Star Hustler”) episodes by his lovably eccentric yet comfortable style of describing the astronomy happening over our heads.  Jack’s episodes are posted to YouTube (like most everything else), so you can get a sample of those classic shows.

The modern version of Star Gazers (note the subtle change in the title from its predecessor, as there are now three hosts for this show) is fantastic!  I highly recommend it for those interested in finding out what astronomical events are taking place each week.  You don’t need to be a degree-carrying scientist to understand the information presented in these episodes either.  It is definitely a good use of five minutes each week!

Jeff Mangum


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Size and Location of the Ecliptic in 600 BC

Question:  Dear Jeff,
I am researching a point about the Ecliptic.
a) if you could ‘draw’ the actual ecliptic on the earth’s surface, what would be its maximum width ?
b)In 600BC at what place approx. on the North coast of Brazil would the ecliptic enter, and where would it exit, South America ?
Thank you very much.  – William

Answer:  Since the Ecliptic is defined as the plane representing the apparent path of the Sun in the sky, it really does not have a “width”.  As for the place where the ecliptic would enter and exit at a location on the north coast of Brazil in 600 BC, what you are really asking is where the Sun will rise and set as viewed from the north coast of Brazil in 600 BC.  Choosing Fortaleza, Brazil, which has a latitude and longitude of (3.7737° S, 38.5748° W), on January 1, 600 BC the Sun would rise and set at azimuth 114 and 246 degrees.  Remember that azimuth is measured positive east from the north, so the rise azimuth is to the south-east (between azimuth 90 and 180) and the set azimuth is to the south-west (between azimuth 180 and 270).

Jeff Mangum

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Faint Dark Bands in the Evening Sky

Question:  Tonight I noticed in the bright evening sky (9pm) over Aberdeenshire a number of faint black bands running across the sky and converging towards the horizon, in a due south direction. They show up very clearly on the photos I have taken and I wonder what is causing them. Many thanks.  – Sarah

Answer:  Since you were looking toward the south around the time when the Milky Way was rising, I wonder if you were seeing the dust bands projected against the higher density of stars in the direction of the disk of our Milky Way Galaxy?  If you want to see what the Milky Way looks like from quite a variety of perspectives, check out the Space.com gallery of Milky Way photos.

Jeff Mangum

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