APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

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APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by APOD Robot » Fri Jul 13, 2018 4:07 am

Image Star Trails and the Bracewell Radio Sundial

Explanation: Sundials use the location of a shadow to measure the Earth's rotation and indicate the time of day. So it's fitting that this sundial, at the Very Large Array Radio Telescope Observatory in New Mexico, commemorates the history of radio astronomy and radio astronomy pioneer Ronald Bracewell. The radio sundial was constructed using pieces of a solar mapping radio telescope array that Bracewell orginaly built near the Stanford University campus. Bracewell's array was used to contribute data to plan the first Moon landing, its pillars signed by visiting scientists and radio astronomers, including two Nobel prize winners. As for most sundials the shadow cast by the central gnomon follows markers that show the solar time of day, along with solstices and equinoxes. But markers on the radio sundial are also laid out according to local sidereal time. They show the position of the invisible radio shadows of three bright radio sources in Earth's sky, supernova remnant Cassiopeia A, active galaxy Cygnus A, and active galaxy Centaurus A. Sidereal time is just star time, the Earth's rotation as measured with the stars and distant galaxies. That rotation is reflected in this composited hour-long exposure. Above the Bracewell Radio Sundial, the stars trace concentric trails around the north celestial pole.

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by RocketRon » Fri Jul 13, 2018 5:32 am

APOD Robot wrote:
Fri Jul 13, 2018 4:07 am
Bracewell's array was used to contribute data to plan the first Moon landing, [/url].
Can anyone expand on this ?

Interesting topic, can't imagine there are too many 'star' dials ?

Certainly solves the problem that the sun doesn't shine at night.
And there would be no analemma/equation of time to complicate things...

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Ann » Fri Jul 13, 2018 6:09 am

RocketRon wrote:
Fri Jul 13, 2018 5:32 am
APOD Robot wrote:
Fri Jul 13, 2018 4:07 am
Bracewell's array was used to contribute data to plan the first Moon landing, [/url].
Can anyone expand on this ?

Interesting topic, can't imagine there are too many 'star' dials ?

Certainly solves the problem that the sun doesn't shine at night.
And there would be no analemma/equation of time to complicate things...


I saw an analemma sun dial when I was in London last week. Can't remember where it was, but perhaps near Saint Paul's Cathedral.

Lost your almanac and your mobile? Can't find one of those newspapers that they hand out for free? You are really lost as to what date it is? No worries! Just find your way to the analemma sun dial in London, watch it at noon and make sure it is a sunny day, and watch the shadow fall onto its figure eight to point you to what day it is! Hmmm, today appears to be the 13th day of July, isn't that interesting?

(The sun dial in London was easier to read. I think it had markings to show off individual days.)

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by De58te » Fri Jul 13, 2018 6:15 am

RocketRon wrote:
Fri Jul 13, 2018 5:32 am
APOD Robot wrote:
Fri Jul 13, 2018 4:07 am
Bracewell's array was used to contribute data to plan the first Moon landing, [/url].
Can anyone expand on this ?

Interesting topic, can't imagine there are too many 'star' dials ?

Certainly solves the problem that the sun doesn't shine at night.
And there would be no analemma/equation of time to complicate things...
From Wikidedia; "At Stanford Professor Bracewell constructed a microwave spectroheliograph (1961), a large and complex radio telescope which produced daily temperature maps of the sun reliably for eleven years, the duration of a solar cycle. The first radio telescope to give output automatically in printed form, and therefore capable of worldwide dissemination by teleprinter, its daily solar weather maps received acknowledgement from NASA for support of the first manned landing on the moon. "

Well, if I recall NASA had two life threatening concerns. Aside from running out of oxygen, failing to enter Moon orbit and shooting past it forever into space, and after Apollo 13, rockets exploding, and burning up in Earth reentry. Well actually there were dozens of concerns, but two concerns regarding space radiation. Sailing through the Van Allen Radiation Belts, and, the Sun emitting a deadly solar flare while the astronauts were on the Moon.

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by MikeSchwab » Fri Jul 13, 2018 8:10 am

Since GPS is easily jamable by a low power transmitter, I was thinking of a design that would use signals from the stars. I.E. pulsars or radio sources. I.E. a vertical stabilized dish with multiple detectors around the edge. Would need 3 sources within ?45? of the vertical at any position on the earth. How big a dish would this require? Any chance of a phase array for planes.

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by rstevenson » Fri Jul 13, 2018 11:40 am

MikeSchwab wrote:
Fri Jul 13, 2018 8:10 am
Since GPS is easily jamable by a low power transmitter, I was thinking of a design that would use signals from the stars. I.E. pulsars or radio sources. I.E. a vertical stabilized dish with multiple detectors around the edge. Would need 3 sources within ?45? of the vertical at any position on the earth. How big a dish would this require? Any chance of a phase array for planes.
I dont think it needs to be stationary. The sky sources it would use for reference are arrayed in an effectively non-moving pattern in the sky and are therefore recognizable no matter the orientation of the receiver. But it would have to take into account its own latitude and longitude, the rotation of the Earth, and a few other factors in order to equal or better the accuracy of the current system. And it would have to get some of that data from another source -- like, maybe, the current GPS system.

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Fri Jul 13, 2018 12:31 pm

https://en.wikipedia.org/wiki/Moondial wrote:
<<Moondials are time pieces similar to a sundial. The most basic moondial, which is identical to a sundial, is only accurate on the night of the full moon. Every night after it becomes an additional (on average) 48 minutes slow, while every night preceding the full moon it is (again on average) 49 minutes fast, assuming there is even enough light to take a reading by. Thus, one week to either side of the full moon the moondial will read 5 hours and 36 minutes before or after the proper time.

More advanced moondials can include charts showing the exact calculations to get the correct time, as well as dials designed with latitude and longitude in mind.

Moondials are very closely associated with lunar gardening (night-blooming plants) and some comprehensive gardening books may mention them.>>
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Fri Jul 13, 2018 1:29 pm

Click to play embedded YouTube video.
rstevenson wrote:
Fri Jul 13, 2018 11:40 am
MikeSchwab wrote:
Fri Jul 13, 2018 8:10 am

Since GPS is easily jamable by a low power transmitter, I was thinking of a design that would use signals from the stars. I.E. pulsars or radio sources.
it would have to take into account its own latitude and longitude, the rotation of the Earth, and a few other factors in order to equal or better the accuracy of the current system. And it would have to get some of that data from another source -- like, maybe, the current GPS system.
Pulsars have millisecond timing structure that might locate you to within ~300 km (provided you can locate those pulsars to within ~300 km ).

Global Very-long-baseline interferometry (VLBI) combined with space-based VLBI antennas such RadioAstron (Spektr-R) can provide angular resolution in microarcseconds. A microarcsecond can pinpoint your own location to ~300 km only if that your sources are closer than ~7 light years (and smaller than ~300 km). (Triangulation doesn't work well with large or distant sources...don't try to locate your position on the road by following the Moon.)

GPS works to accurately locate us to within meters only because we can accurately locate the GPS sources, themselves, to within meters.
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Ann » Fri Jul 13, 2018 2:32 pm

neufer wrote:
Fri Jul 13, 2018 1:29 pm

Pulsars have millisecond timing structure that might locate you to within ~300 km (provided you can locate those pulsars to within ~300 km ).

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Chris Peterson » Fri Jul 13, 2018 5:45 pm

MikeSchwab wrote:
Fri Jul 13, 2018 8:10 am
Since GPS is easily jamable by a low power transmitter, I was thinking of a design that would use signals from the stars. I.E. pulsars or radio sources. I.E. a vertical stabilized dish with multiple detectors around the edge. Would need 3 sources within ?45? of the vertical at any position on the earth. How big a dish would this require? Any chance of a phase array for planes.
Actually, it's not that easy to jam GPS. The reason is because it uses spread spectrum radio sources, which allow the signal to be extracted when the S/N is very poor and which mean that single frequency jammers don't work well. The problem with using an astronomical radio source is that it's just noise, there is no signal that can be used for correlation. So you basically need a radio telescope to detect the source, as opposed to a postage stamp sized antenna in a handheld box.

It is common in science fiction stories where starships have some kind of jump drives to use the known constellation of pulsars to determine position in very much the same way that GPS works, however.
Chris

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by fcastle69 » Fri Jul 13, 2018 7:43 pm

Chris Peterson wrote:
Fri Jul 13, 2018 5:45 pm

Actually, it's not that easy to jam GPS.
Depends if you think spending money is easy or not.

https://www.thesignaljammer.com/categories/GPS-Jammers/

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Fri Jul 13, 2018 9:00 pm

Chris Peterson wrote:
Fri Jul 13, 2018 5:45 pm

It is common in science fiction stories where starships have some kind of jump drives to use the known constellation of pulsars to determine position in very much the same way that GPS works,
https://en.wikipedia.org/wiki/Pioneer_plaque wrote:
<<The Pioneer plaques are a pair of gold-anodized aluminium plaques which were placed on board the 1972 Pioneer 10 and 1973 Pioneer 11 spacecraft, featuring a pictorial message, in case either Pioneer 10 or 11 is intercepted by extraterrestrial life. The plaques show the nude figures of a human male and female along with several symbols that are designed to provide information about the origin of the spacecraft. The plaques were attached to the spacecraft's antenna support struts in a position that would shield them from erosion by interstellar dust.

The radial pattern on the left of the plaque shows 15 lines emanating from the same origin. Fourteen of the lines have corresponding long binary numbers, which stand for the periods of pulsars, using the hydrogen spin-flip transition frequency as the unit. Since these periods will change over time, the epoch of the launch can be calculated from these values.

The lengths of the lines show the relative distances of the pulsars to the Sun. A tick mark at the end of each line gives the Z coordinate perpendicular to the galactic plane.

If the plaque is found, only some of the pulsars may be visible from the location of its discovery. Showing the location with as many as 14 pulsars provides redundancy so that the location of the origin can be triangulated even if only some of the pulsars are recognized.

The data for one of the pulsars is misleading. When the plaque was designed, the frequency of pulsar "1240" (now known as J1243-6423) was known to only three significant decimal digits: 0.388 second. The map lists the period to much greater precision: 100000110110010110001001111000. Rounding this off at about 10 significant bits (100000110100000000000000000000) would have provided a hint of this uncertainty. This pulsar is represented by the long line pointing down and to the right.

The fifteenth line on the plaque extends to the far right, behind the human figures. This line indicates the Sun's relative distance to the center of the galaxy.>>
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by MarkBour » Fri Jul 13, 2018 9:02 pm

If I understand the description of the Bracewell Radio Sundial correctly, it is really mainly a sundial, fairly standard in its manner, plotting the shadow of our Sun and using it to give the date and time. But as the caption points out, there are intriguing markings on the floor of this sundial that highlight an alternative means of determining the (sidereal) time, based on the direction of some bright radio sources. And other commenters, such as RocketRon, MikeSchwab, and rstevenson have made some fascinating suggestions along this line (please excuse that pun).

What would they call the device if it actually used those "bright radio sources" to tell the time? A radio dial ? (RocketRon suggested "star dial", but I'm not sure he meant a radio source for that term.)

And how would that best work? To see Cassiopeia A (listed as "the brightest extrasolar radio source in the sky above 1GHz"), what do you need? Can you comfortably observe and find the direction of Cassiopeia A with a device that is less than 5 meters in diameter?
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by MarkBour » Fri Jul 13, 2018 9:15 pm

When the Morgotheans get to our fair planet, they are going to be very hungry and even more angry.

"False advertising!" they will cry. "We saw a floating space advertisement indicating that there was a straight super-highway path clear from the Galactic center out to this point, where one could easily obtain delicious humanoids of two varieties at fast-food stations! I am literally starving."
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Chris Peterson » Fri Jul 13, 2018 10:17 pm

fcastle69 wrote:
Fri Jul 13, 2018 7:43 pm
Chris Peterson wrote:
Fri Jul 13, 2018 5:45 pm

Actually, it's not that easy to jam GPS.
Depends if you think spending money is easy or not.

https://www.thesignaljammer.com/categories/GPS-Jammers/
They don't work very well, and they work especially poorly on military GPS units. But in any case, if you don't have a modulated radio source to work with, there's no practical way to use that source for locating yourself.
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Fri Jul 13, 2018 11:32 pm

Click to play embedded YouTube video.
MarkBour wrote:
Fri Jul 13, 2018 9:15 pm

When the Morgotheans get to our fair planet, they are going to be very hungry and even more angry.

"False advertising!" they will cry. "We saw a floating space advertisement indicating that there was a straight super-highway path clear from the Galactic center out to this point, where one could easily obtain delicious humanoids of two varieties at fast-food stations! I am literally starving."

"Well, if you wanted to make Serak the
Preparer cry... mission accomplished!"
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by heehaw » Sat Jul 14, 2018 12:47 am

My goodness, we have had about a million star-swirl APODS, and yet we get this huge outpouring over this one! What's so special?

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Fred the Cat » Sat Jul 14, 2018 1:45 am

heehaw wrote:
Sat Jul 14, 2018 12:47 am
My goodness, we have had about a million star-swirl APODS, and yet we get this huge outpouring over this one! What's so special?
Because if we ever detect micro-radio waves at least we'll know they're civilized. :wink:
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Sat Jul 14, 2018 2:13 am

Fred the Cat wrote:
Sat Jul 14, 2018 1:45 am
heehaw wrote:
Sat Jul 14, 2018 12:47 am

My goodness, we have had about a million star-swirl APODS, and yet we get this huge outpouring over this one! What's so special?
Because if we ever detect micro-radio waves at least we'll know they're civilized. :wink:
  • It all has to do with the tremendous interest in UFO's, pulsars & the Shakespeare Authorship Question:
https://aas.org/obituaries/ronald-n-bracewell-1921-2007 wrote:
Vahé Petrosian (Stanford University) wrote:
<<Ronald N. Bracewell, Professor Emeritus (since 1991) of Electrical Engineering at Stanford University, and a true renaissance man of science, died of a heart attack on 12 August 2007 at his home.

...I would like to thank (Stanford) emeritus professors Peter Sturrock (et al) for valuable advice and suggestions.
>>
https://en.wikipedia.org/wiki/Peter_A._Sturrock wrote:
<<Peter Andrew Sturrock (born 20 March 1924) is a British scientist and emeritus professor of applied physics at Stanford University. Much of Sturrock's career has been devoted to astrophysics, plasma physics, and solar physics, but Sturrock is interested in other fields, including ufology, scientific inference, the history of science, and the philosophy of science.

Sturrock noted that many scientists wished to see UFOs discussed in scientific journals. He subsequently helped establish the Society for Scientific Exploration in 1982 to give a scientific forum to subjects that are neglected by the mainstream. Their publication, the Journal of Scientific Exploration, has been published since 1987. Sturrock subsequently wrote the 2000 book The UFO Enigma: A New Review of the Physical Evidence.

In 2013, Sturrock published _AKA Shakespeare: A Scientific Approach to the Authorship Question_. In this book, he lays out a method for weighing evidence which he developed for studying pulsars. Sturrock then invites the reader to apply the method to tabulate their own "degree of belief" in three different candidates for authorship of the works usually attributed to Shakespeare.>>
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by MarkBracewell » Sat Jul 14, 2018 4:18 am

Dad liked sundials. The folks who made the radio sundial happen knew this, Miller Goss, Bob Lash, Woody Harrelson. This one at Stanford I built for him. He made one for me just a week before he passed away. He was a good egg. Image

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Sat Jul 14, 2018 11:49 am

MarkBracewell wrote:
Sat Jul 14, 2018 4:18 am

Image
Dad liked sundials. The folks who made the radio sundial happen knew this, Miller Goss, Bob Lash, Woody Harrelson. This one at Stanford I built for him. He made one for me just a week before he passed away. He was a good egg.
http://www.waymarking.com/waymarks/WMH5D4_Sundial_Huang_Engineering_Center_Stanford_CA wrote:
CÆLUM SCRUTANDO LEGES MOTUS DIDICIMUS
We learnt the laws of motion by studying the heavens

<<The mural sundial at Stanford is descended from a solar clock that once graced the Tower of the Winds in the Agora in Athens. This one has an analemma for each hour, color coded for season, showing time that is accurate to the minute without correction for date. You can set your watch by it.

Each day on the hour the circular spot of sunlight thrown by the oculus will fall on the analemma for that hour and will tell the season, sap green for Spring, cherry red for Summer, autumn gold for Fall, and ice blue for Winter. At the equinox the spot of light moves up the straight line marked ÆQUINOX. At the Summer solstice the spot moves up the hyperbola labeled SOLSTICE; at the Winter solstice the spot traces out the upper hyperbola.

The Latin motto reminds us that Newton's three laws that govern everything that moves originated in astronomy. A second motto, d/dt ≠ 0 signifies, in the notation of the philosopher Leibnitz, that time does not stand still - an admonition to us all that time is precious.

The sundial project was initiated by Professor Robert H. Bracewell and supported by Dean James F. Gibbons. The sundial plate was hand carved by Mark Bracewell and hung under the auspices of Dean John L. Hennessy. >>
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by rstevenson » Sat Jul 14, 2018 4:28 pm

neufer wrote:
Sat Jul 14, 2018 2:13 am
https://en.wikipedia.org/wiki/Peter_A._Sturrock wrote: ...
In 2013, Sturrock published _AKA Shakespeare: A Scientific Approach to the Authorship Question_. In this book, he lays out a method for weighing evidence which he developed for studying pulsars. Sturrock then invites the reader to apply the method to tabulate their own "degree of belief" in three different candidates for authorship of the works usually attributed to Shakespeare.>>
Using a method developed for studying physical phenomena to weigh evidence about human behaviour strikes me as not a sound scientific approach. Pulsars are not known to hide evidence, create "evidence", engage in willful avoidance of evidence, or in any other way muddy the waters -- they just are. Humans, unfortunately, can and will do all of those things, so other methods are likely to yield better results.

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by neufer » Sat Jul 14, 2018 6:00 pm

rstevenson wrote:
Sat Jul 14, 2018 4:28 pm
neufer wrote:
Sat Jul 14, 2018 2:13 am
https://en.wikipedia.org/wiki/Peter_A._Sturrock wrote: ...
In 2013, Sturrock published _AKA Shakespeare: A Scientific Approach to the Authorship Question_. In this book, he lays out a method for weighing evidence which he developed for studying pulsars. Sturrock then invites the reader to apply the method to tabulate their own "degree of belief" in three different candidates for authorship of the works usually attributed to Shakespeare.>>
Using a method developed for studying physical phenomena to weigh evidence about human behaviour strikes me as not a sound scientific approach. Pulsars are not known to hide evidence, create "evidence", engage in willful avoidance of evidence, or in any other way muddy the waters -- they just are. Humans, unfortunately, can and will do all of those things, so other methods are likely to yield better results.
  • Humans, unfortunately, are necessary for interpreting pulsar signals based upon
    imperfect a posteriori models (e.g., little green men, gravitational radiation, star quakes, planets, etc.)

    Sturrock used his own version of subjective Bayesian probability to select between:
    • imperfect pulsar models ... and later:
      three alternative Shakespeare canon candidates.
https://en.wikipedia.org/wiki/Thomas_Bayes wrote:
<<Bayesian probability is the name given to several related interpretations of probability as an amount of epistemic confidence – the strength of beliefs, hypotheses etc. –, rather than a frequency. This allows the application of probability to all sorts of propositions rather than just ones that come with a reference class. Since its rebirth in the 1950s, advancements in computing technology have allowed scientists from many disciplines to pair traditional Bayesian statistics with random walk techniques. The use of the Bayes theorem has been extended in science and in other fields.

Thomas Bayes (c. 1701 – 7 April 1761) himself might not have embraced the broad interpretation now called Bayesian, which was in fact pioneered and popularised by Pierre-Simon Laplace. Bayes defines probability of an event as "the ratio between the value at which an expectation depending on the happening of the event ought to be computed, and the value of the thing expected upon its happening". This is a subjective definition, and does not require repeated events; however, it does require that the event in question be observable, for otherwise it could never be said to have "happened".>>
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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Cousin Ricky » Wed Jul 18, 2018 3:31 pm

This APOD got me wondering: do astronomers maintain an alternate calendar based on sidereal days?

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Re: APOD: Star Trails and the Bracewell Radio... (2018 Jul 13)

Post by Chris Peterson » Wed Jul 18, 2018 3:43 pm

Cousin Ricky wrote:
Wed Jul 18, 2018 3:31 pm
This APOD got me wondering: do astronomers maintain an alternate calendar based on sidereal days?
No. The standard astronomical calendar is based on Julian Day, which is an integer count of solar days.
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