hdate: how to get output for a specific date?

[Shmu26]

Let's say I want to get sunrise time or other output for tomorrow, or for a certain date.
how to do that in linux terminal?

[Boruch Baum]

On 2021-02-17 08:46, Shmuel Globus wrote:

Let's say I want to get sunrise time or other output for tomorrow, or
for a certain date.
how to do that in linux terminal?

Take a look at the program's documentation for all of its features. If
you feel that the feature you're asking for isn't clearly described,
then let me know. You had me worried for a bit, but I double-checked and
your question is very clearly answered both by 'man hdate' and
'hdate --help'.

Kol tuv,

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[Shmu26]

You are right. It's there. To get sunrise and sunset times for tomorrow, the 18th of February 2021, I did
hdate 18 2 2021 -s --sun
and it worked perfectly.

[Boruch Baum]

On 2021-02-17 09:48, Shmu26 wrote:

You are right. It's there. To get sunrise and sunset times for
tomorrow, the 18th of February 2021, I did
hdate 18 2 2021 -s --sun
and it worked perfectly.

Great. Now just be certain that you're getting data for the correct
location on Earth. If the program hasn't correctly guessed your
location, you need to explicitly define it. Again, see the documentation
for details.

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[Shmu26]

Yup, did that. It is explained very nicely in the config file, and I think the warning also displays in the terminal until it is configured. Thanks for the great app!

[Shmu26]

So here comes a perhaps unreasonable question: Can I configure the sunrise time to display as eight minutes earlier than the standard output? My minyan thinks that the sun rises 8 minutes earlier than the time posted by hdate for my locale (Jerusalem, Israel).
For reference, my hdaterc file says as follows. I think this is right for Jerusalem.

LATITUDE=31
LONGITUDE=34
# TIMEZONE may may be in decimal format or in the form degrees[:minutes]
# with the characters :'" as possible delimiters.
TIMEZONE=2

[Boruch Baum]

On 2021-02-21 14:43, Shmu26 wrote:

My minyan thinks that the sun rises 8 minutes earlier than the
time posted by hdate for my locale (Jerusalem, Israel).

You minyan could be correct for its exact point on Earth and the
decision of your local posek should always be respected. The computer
program doesn't guess exactly where within J-m you are, or whether your
location has its horizon sunrise/sunset obstructed by some
hill/mountain; there will be variation accordingly. Also be aware that
your posek may be intentionally 'padding' the numbers לחומרא, ie.

מקדים כמה דקות ע"ש ומאחר כמה דקות מוצ"ש

For reference, my hdaterc file says as follows. I think this is right
for Jerusalem.

LATITUDE=31
LONGITUDE=34

Hebrew-language Wikipedia claims the following coordinates:

Har Nof 31°47′05″N 35°10′30″E
Pisgat Zeev 31°49′30″N 35°14′30″E

And just for comparison:

Maale Adumim 31°46′46″N 35°18′20″E

The one degree difference in longitude between your 34 and my 35 will
already account for five minutes of discrepancy, which will leave you
with just a three minute discrepancy to work with, which be
posek-padding or due to sun-block. Ask what exact coordinates they are
using (if they are using a modern computerized shul clock, it may
display the exact coordinates at some point ... AND it may be using this
software internally ...).

If that isn't enough to align the numbers to your satisfaction, find out
whether the difference is consistent throughout the year, and make a
personal decision to either: 1) remember the three minute difference
yourself; 2) give the computer program an 'adjusted' longitude to match
your posek.

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[Shmu26]

Yeah, it's all about altitude and taking the mountains on the horizon into account. The times they use in the minyan are based on actual sightings of the sunrise, which of course depends a lot on where the sighter was standing.
So your last idea, to choose a longitude that gives me the result I want, sounds right. It will be interesting to see if it stays accurate as the seasons change.
Thanks!

[Boruch Baum]

On 2021-02-21 18:12, Shmu26 wrote:

Yeah, it's all about altitude and taking the mountains on the horizon
into account. The times they use in the minyan are based on actual
sightings of the sunrise, which of course depends a lot on where the
sighter was standing.
So your last idea, to choose a longitude that gives me the result I
want, sounds right.

Uh, not so fast... In the situation you describe, if you adjust the
computer program's longitude configuration to account for the sunrise
obstruction, the computer program will apply the same adjustment for
sunset, which will only be valid if the shul has a similar second
obstruction to the West. There are also (usually very quantitatively
small) nafka mina for the length of a sha'a zmania and therefore any
calculation based upon that (In your case, if the sunrise adjustment is
three minutes, then the sha'a zmania adjustment would be 3/12 minutes,
ie. 15 seconds per hour.

This is a limitation of the computer program, and a future version
(whenever/if-ever) could add two configuration options for independent
adjustments. It hasn't been done because it's something very prone to
variation throughout the year in a way a computer program can't foresee
without access to detailed GIS data ....

It will be interesting to see if it stays accurate as the seasons
change.

You can pretty easily check for yourself without needing to wait. There
is a ~46 degree variance in the path of the sun over the course of a
half-year. For this purpose, since we're discussing the sun, it's
convenient / appropriate to use their calendar: ~December 21 is the
'Winter Solstice' when the sun travels at its Southern-most path, ~June
21 is the 'Summer Solstice' when the sun travels along its Northern-most
path, and ~March 20 is the 'Vernal Equinox' when the sun travels
straight down the middle. So, at Netz on ~20 March, if for some reason
you aren't davening vatikin, note the point of sunrise and compare the
height / angle of the obstruction at that point to ~23 degrees South and
~23 degrees North (someone with a 'smart phone' can help you by using
the phone's compass app to point the 23 degrees). On any other day of
the year, you can check an astronomy table for where along the 46 degree
route is the sun.

Thanks!

Quite welcome. Kol Tuv.

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[Shmu26]

Thanks for the heads up about the sunset time changing accordingly. I did notice that inexorable side-effect.