When Does the Earth’s Prime Meridian Line Up With the Celestial Meridian?

Question: On what date and time to the nearest second (in the past) has earths prime meridian directly lined up and faced the
“celestial meridian” and I am talking about using the “equatorial coordinate system”

Definition for “Celestial Meridian” which I am using from Dictionary.com below:
“In the equatorial coordinate system, a great circle on the celestial sphere passing through the celestial poles and the vernal equinox. It represents the zero point for the horizontal coordinate in this system, having a right ascension of 0 hours.”

I might have thought it was vernal equinox but that happens once every year but the earth doesnt rotate exactly 360 degrees per year so the prime meridian on earth would not always line up. So thats not my answer so I cant just look up Vernal equinox dates on the web.
Its might happen every 4 vernal equinox but not sure. That still would not give me a date which I am looking for.

Prefer UT time or Julian date but anything would help.
Does the date time I am looking for even have a unique name for it?

– Steve


Answer: The Earth’s “prime meridian” is the great circle which passes through the Earth’s poles and represents the start of the longitude coordinate system, or zero-degrees longitude.  Comparing that to the definition you have given above for the celestial meridian, I would say that these two meridians line up once each day.  The exact time in a day that the prime meridian aligns with the celestial meridian is determined by the difference between a solar and sidereal day, or about 4 minutes.

Jeff Mangum

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What Are Those Two Bright Lights on the Western Horizon?

Question: On July 15, 2015, observing from Las Vegas, Nevada, there are 2 objects (Low) in the Western sky. One is bright the other dim. Are they Venus and Jupiter?  – George

Answer: Yes!  Venus, the brighter of the two, is to the south (left when facing west) of the other bright guy, Jupiter.

Jeff Mangum

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What is that Bright Object in the Western Sky After Sunset?

Question: What bright light is in nepa’s sw sky at 30degrees above the horizion at 9:15 edt.  – Tom

Answer: Although I don’t exactly know what location you are observing from, I suspect that you are seeing Jupiter and Venus, which are both currently visible on the western horizon after sunset.

Jeff Mangum

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Are UFO’s Real?

Question: What is your opion of UFO’s  observed from ground surface and those observed from space stations and shuttle flights?  I have heard the there is a “Coloney” on the side of the moon we never seen.  Your opinion?  – Kevin

Answer: The fact is that there has never been a confirmed sighting of a UFO that has also been confirmed as evidence of an extraterrestrial presence.  Conspiracy theories are usually based on little or no hard evidence, and are often simply designed to entertain.

Jeff Mangum

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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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