Pyephem 15 minutes discrepancy when manually computing equinoxes and solstices
the code below tries to compute manually the first equinox of 2019.
It returns
('d1=', 2019/3/20 21:43:48)
('d2=', 2019/3/20 21:43:49)
2019/3/20 21:58:31
that is, a discrepancy of 15 minutes with the real equinox. Is this normal?
Did I forget something? The problem also occurs with the solstices, and also if I used the integrated newton method. Could it have something to do with the epoch of computation?
Thanks,
Dennis
import ephem
sun = ephem.Sun()
# computing Spring equinox:
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('180.0')
for i in range(20):
#middle date
d3=(d1+d2)/2
sun.compute(d3)
if sun.hlon>a:
d2=d3
else:
d1=d3
print("d1=",ephem.Date(d1))
print("d2=",ephem.Date(d2))
d1 = ephem.next_equinox('2019')
print(d1)
python datetime pyephem
asked Jan 16 at 0:15
DennisDennis
62
add a comment |
the code below tries to compute manually the first equinox of 2019.
It returns
('d1=', 2019/3/20 21:43:48)
('d2=', 2019/3/20 21:43:49)
2019/3/20 21:58:31
that is, a discrepancy of 15 minutes with the real equinox. Is this normal?
Did I forget something? The problem also occurs with the solstices, and also if I used the integrated newton method. Could it have something to do with the epoch of computation?
Thanks,
Dennis
import ephem
sun = ephem.Sun()
# computing Spring equinox:
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('180.0')
for i in range(20):
#middle date
d3=(d1+d2)/2
sun.compute(d3)
if sun.hlon>a:
d2=d3
else:
d1=d3
print("d1=",ephem.Date(d1))
print("d2=",ephem.Date(d2))
d1 = ephem.next_equinox('2019')
print(d1)
python datetime pyephem
asked Jan 16 at 0:15
DennisDennis
62
Sorry, read Summer solstice instead of Summer equinox :-(
– Dennis
Jan 16 at 0:17
1
You can click on the edit link below your post to update your post :)
– Gino Mempin
Jan 16 at 0:42
The problem also occurs when using Newton's method within pyephem:
– Dennis
Jan 16 at 13:58
add a comment |
the code below tries to compute manually the first equinox of 2019.
It returns
('d1=', 2019/3/20 21:43:48)
('d2=', 2019/3/20 21:43:49)
2019/3/20 21:58:31
that is, a discrepancy of 15 minutes with the real equinox. Is this normal?
Did I forget something? The problem also occurs with the solstices, and also if I used the integrated newton method. Could it have something to do with the epoch of computation?
Thanks,
Dennis
import ephem
sun = ephem.Sun()
# computing Spring equinox:
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('180.0')
for i in range(20):
#middle date
d3=(d1+d2)/2
sun.compute(d3)
if sun.hlon>a:
d2=d3
else:
d1=d3
print("d1=",ephem.Date(d1))
print("d2=",ephem.Date(d2))
d1 = ephem.next_equinox('2019')
print(d1)
python datetime pyephem
asked Jan 16 at 0:15
DennisDennis
62
the code below tries to compute manually the first equinox of 2019.
It returns
('d1=', 2019/3/20 21:43:48)
('d2=', 2019/3/20 21:43:49)
2019/3/20 21:58:31
that is, a discrepancy of 15 minutes with the real equinox. Is this normal?
Did I forget something? The problem also occurs with the solstices, and also if I used the integrated newton method. Could it have something to do with the epoch of computation?
Thanks,
Dennis
import ephem
sun = ephem.Sun()
# computing Spring equinox:
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('180.0')
for i in range(20):
#middle date
d3=(d1+d2)/2
sun.compute(d3)
if sun.hlon>a:
d2=d3
else:
d1=d3
print("d1=",ephem.Date(d1))
print("d2=",ephem.Date(d2))
d1 = ephem.next_equinox('2019')
print(d1)
python datetime pyephem
python datetime pyephem
asked Jan 16 at 0:15
DennisDennis
62
asked Jan 16 at 0:15
DennisDennis
62
edited Jan 18 at 22:01
Dennis
asked Jan 16 at 0:15
DennisDennis
62
asked Jan 16 at 0:15
DennisDennis
62
asked Jan 16 at 0:15
DennisDennis
62
62
Sorry, read Summer solstice instead of Summer equinox :-(
– Dennis
Jan 16 at 0:17
1
You can click on the edit link below your post to update your post :)
– Gino Mempin
Jan 16 at 0:42
The problem also occurs when using Newton's method within pyephem:
– Dennis
Jan 16 at 13:58
add a comment |
Sorry, read Summer solstice instead of Summer equinox :-(
– Dennis
Jan 16 at 0:17
1
You can click on the edit link below your post to update your post :)
– Gino Mempin
Jan 16 at 0:42
The problem also occurs when using Newton's method within pyephem:
– Dennis
Jan 16 at 13:58
Sorry, read Summer solstice instead of Summer equinox :-(
– Dennis
Jan 16 at 0:17
Sorry, read Summer solstice instead of Summer equinox :-(
– Dennis
Jan 16 at 0:17
1
1
You can click on the edit link below your post to update your post :)
– Gino Mempin
Jan 16 at 0:42
You can click on the edit link below your post to update your post :)
– Gino Mempin
Jan 16 at 0:42
The problem also occurs when using Newton's method within pyephem:
– Dennis
Jan 16 at 13:58
The problem also occurs when using Newton's method within pyephem:
– Dennis
Jan 16 at 13:58
add a comment |
2 Answers
2
active
oldest
votes
It looks like the difference is because PyEphem's underlying astronomy library always measures heliocentric longitude relative to the coordinates of J2000, which by the date you are asking about is noticeably different from the coordinates-of-date which are used to define the equinox.
Try running this as your compute step:
sun.compute(d3, epoch=d3)
and then look for when sun.ra is zero degrees; the result should be the equinox. I'll see about getting the PyEphem Quick Reference updated to note that heliocentric coordinates don't seem to pay attention to the epoch= parameter.
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
add a comment |
Many thanks, Brandon, this is very helpful and I am finally getting the correct value! In fact, it seems that the equinoxes are defined by the right ascension being equal to 0h, 6h, 12h, 18h, and not the heliocentric longitude being 0, 90, 180, 270. There is a slight difference between ra and hlon, when you run the code below. But this leads to another question. The Wikipedia page https://en.wikipedia.org/wiki/Equinox says that the equinoxes are defined by the longitude being 0 or 180. So who is correct?
import ephem
sun = ephem.Sun()
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('0.0') # or 90, or 180, or 270
def spring_equinox(date):
sun.compute(date)
return ephem.degrees(sun.ra - a).znorm
d = ephem.newton(spring_equinox, d1, d2)
print(ephem.Date(d))
print sun.ra
print sun.hlon
answered Jan 20 at 16:27
DennisDennis
62
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
StackExchange.using("externalEditor", function () {
StackExchange.using("snippets", function () {
StackExchange.snippets.init();
});
});
}, "code-snippets");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "1"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f54208690%2fpyephem-15-minutes-discrepancy-when-manually-computing-equinoxes-and-solstices%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
It looks like the difference is because PyEphem's underlying astronomy library always measures heliocentric longitude relative to the coordinates of J2000, which by the date you are asking about is noticeably different from the coordinates-of-date which are used to define the equinox.
Try running this as your compute step:
sun.compute(d3, epoch=d3)
and then look for when sun.ra is zero degrees; the result should be the equinox. I'll see about getting the PyEphem Quick Reference updated to note that heliocentric coordinates don't seem to pay attention to the epoch= parameter.
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
add a comment |
It looks like the difference is because PyEphem's underlying astronomy library always measures heliocentric longitude relative to the coordinates of J2000, which by the date you are asking about is noticeably different from the coordinates-of-date which are used to define the equinox.
Try running this as your compute step:
sun.compute(d3, epoch=d3)
and then look for when sun.ra is zero degrees; the result should be the equinox. I'll see about getting the PyEphem Quick Reference updated to note that heliocentric coordinates don't seem to pay attention to the epoch= parameter.
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
add a comment |
It looks like the difference is because PyEphem's underlying astronomy library always measures heliocentric longitude relative to the coordinates of J2000, which by the date you are asking about is noticeably different from the coordinates-of-date which are used to define the equinox.
Try running this as your compute step:
sun.compute(d3, epoch=d3)
and then look for when sun.ra is zero degrees; the result should be the equinox. I'll see about getting the PyEphem Quick Reference updated to note that heliocentric coordinates don't seem to pay attention to the epoch= parameter.
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
It looks like the difference is because PyEphem's underlying astronomy library always measures heliocentric longitude relative to the coordinates of J2000, which by the date you are asking about is noticeably different from the coordinates-of-date which are used to define the equinox.
Try running this as your compute step:
sun.compute(d3, epoch=d3)
and then look for when sun.ra is zero degrees; the result should be the equinox. I'll see about getting the PyEphem Quick Reference updated to note that heliocentric coordinates don't seem to pay attention to the epoch= parameter.
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
answered Jan 19 at 5:09
Brandon RhodesBrandon Rhodes
52k1391128
52k1391128
add a comment |
add a comment |
Many thanks, Brandon, this is very helpful and I am finally getting the correct value! In fact, it seems that the equinoxes are defined by the right ascension being equal to 0h, 6h, 12h, 18h, and not the heliocentric longitude being 0, 90, 180, 270. There is a slight difference between ra and hlon, when you run the code below. But this leads to another question. The Wikipedia page https://en.wikipedia.org/wiki/Equinox says that the equinoxes are defined by the longitude being 0 or 180. So who is correct?
import ephem
sun = ephem.Sun()
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('0.0') # or 90, or 180, or 270
def spring_equinox(date):
sun.compute(date)
return ephem.degrees(sun.ra - a).znorm
d = ephem.newton(spring_equinox, d1, d2)
print(ephem.Date(d))
print sun.ra
print sun.hlon
answered Jan 20 at 16:27
DennisDennis
62
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
add a comment |
Many thanks, Brandon, this is very helpful and I am finally getting the correct value! In fact, it seems that the equinoxes are defined by the right ascension being equal to 0h, 6h, 12h, 18h, and not the heliocentric longitude being 0, 90, 180, 270. There is a slight difference between ra and hlon, when you run the code below. But this leads to another question. The Wikipedia page https://en.wikipedia.org/wiki/Equinox says that the equinoxes are defined by the longitude being 0 or 180. So who is correct?
import ephem
sun = ephem.Sun()
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('0.0') # or 90, or 180, or 270
def spring_equinox(date):
sun.compute(date)
return ephem.degrees(sun.ra - a).znorm
d = ephem.newton(spring_equinox, d1, d2)
print(ephem.Date(d))
print sun.ra
print sun.hlon
answered Jan 20 at 16:27
DennisDennis
62
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
add a comment |
Many thanks, Brandon, this is very helpful and I am finally getting the correct value! In fact, it seems that the equinoxes are defined by the right ascension being equal to 0h, 6h, 12h, 18h, and not the heliocentric longitude being 0, 90, 180, 270. There is a slight difference between ra and hlon, when you run the code below. But this leads to another question. The Wikipedia page https://en.wikipedia.org/wiki/Equinox says that the equinoxes are defined by the longitude being 0 or 180. So who is correct?
import ephem
sun = ephem.Sun()
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('0.0') # or 90, or 180, or 270
def spring_equinox(date):
sun.compute(date)
return ephem.degrees(sun.ra - a).znorm
d = ephem.newton(spring_equinox, d1, d2)
print(ephem.Date(d))
print sun.ra
print sun.hlon
answered Jan 20 at 16:27
DennisDennis
62
Many thanks, Brandon, this is very helpful and I am finally getting the correct value! In fact, it seems that the equinoxes are defined by the right ascension being equal to 0h, 6h, 12h, 18h, and not the heliocentric longitude being 0, 90, 180, 270. There is a slight difference between ra and hlon, when you run the code below. But this leads to another question. The Wikipedia page https://en.wikipedia.org/wiki/Equinox says that the equinoxes are defined by the longitude being 0 or 180. So who is correct?
import ephem
sun = ephem.Sun()
d1 = ephem.Date('2019/03/15')
d2 = ephem.Date('2019/03/25')
a=ephem.degrees('0.0') # or 90, or 180, or 270
def spring_equinox(date):
sun.compute(date)
return ephem.degrees(sun.ra - a).znorm
d = ephem.newton(spring_equinox, d1, d2)
print(ephem.Date(d))
print sun.ra
print sun.hlon
answered Jan 20 at 16:27
DennisDennis
62
answered Jan 20 at 16:27
DennisDennis
62
answered Jan 20 at 16:27
DennisDennis
62
answered Jan 20 at 16:27
DennisDennis
62
62
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
add a comment |
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
(Before someone else points it out: this might have fit better as an update to your main question, maybe by editing and having it in a section called "Follow up:".) The RA and longitude should be 0 at the same time. The difference is that PyEphem is willing to give you RA in the coordinate system of the date you asked about, but longitude only in J2000 coordinates. Try reading about "precession", maybe on the Wikipedia, to understand why the coordinate systems shift where 0 is every year.
– Brandon Rhodes
Jan 20 at 21:16
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
Thanks. I know about precession, it's just that I didn't realize that RA and longitude can use different epochs! Many thanks again.
– Dennis
Jan 21 at 10:49
add a comment |
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f54208690%2fpyephem-15-minutes-discrepancy-when-manually-computing-equinoxes-and-solstices%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Sorry, read Summer solstice instead of Summer equinox :-(
– Dennis
Jan 16 at 0:17
1
You can click on the edit link below your post to update your post :)
– Gino Mempin
Jan 16 at 0:42
The problem also occurs when using Newton's method within pyephem:
– Dennis
Jan 16 at 13:58