# Time to reinvent clock

Home Forums Bais Medrash Time to reinvent clock

Viewing 21 posts - 1 through 21 (of 21 total)
• Author
Posts
• #1347891

HaLeiVi
Participant

My uncle once noticed that on the calendar in Reb Moshe Feinstein’s Yeshiva the size of the Sha’ah Zmanis was different before Chatzos than after Chatzos on the same day. Thinking this was a mistake, he brought it to Reb Moshe’s attention.

Reb Moshe responded that he made this calendar himself and it is not a mistake. He explained that the Chatzos is not calculated as midrange between Netz and Shkia, but rather as the moment of the sun’s transit. Separately, we calculate the times of Alos/Netz and Tzes/Shkia. Then we divide the time from each to the Chatzos to get the size of the Sha’os Zmanios.

Since at times the spans of Netz to Chatzos and Chatzos to Shkia are not equal, the size of the Sha’os Zmanios are also not equal. Ad Kan.

Now, this makes me wonder. Once we accept this, why not take it to the next obvious step, that no two hours are the same. Obviously on an actual sundial, if the last hours of the day are shorter than the first hours, this would be happening gradually.

And being that for many years we were going by an actual sundial, which is what Sha’ah Zmanis is based on, shouldn’t we adjust all our Zmanim to the natural schedule? Why bother fixing six hours to the same length when it doesn’t reflect reality.

#1347925

JJ2020
Participant

When they announce the mold in shul they also round the chalakim. Maybe it’s just for simplicity.

#1347937

iacisrmma
Participant

It seems R’ Moshe disagrees with the Mishna Berurah who clearly states how the Shar Zmanios are calculates as being 12 equal parts.

#1347941

Participant

JJ2020

“They also round the chalakim” ???

Whatever does that mean?

#1347963

Participant

HaLeiVi, I think you are making two separate mistakes here.

Firstly, it is a common misconception that sundials indicate shaos zemanios. Actually, most sundials indicate regular hours since they are based on the hour angle of the sun around a north-south axis. While sundials can also be made to indicate shaos zemanios (at least when calculated from netz to shekia), this is complicated to achieve accurately throughout the year, and was not common. Wikipedia’s article on sundials is a bit short on information about this particular point, but there are other websites devoted to sundials where more info can be found.

Secondly, R’ Moshe’s shittah about different size hours before and after midday is not based on any gradual astronomical phenomenon that could be conceivably indicated on any kind of sundial. It is based on the fact that the day starts at alos hashachar which is far longer before sunrise than halachic nightfall is after sunset. Thus the day is lopsided, and halfway through it is before midday (defined as when the sun is at the highest point of it’s path). Rather than redefine halachic midday as being before astronomic midday, R’ Moshe redefines the hours, making a morning sha’a zemanis a sixth of the time between alos hashachar and midday, and an afternoon sha’a zemanis a sixth of the time between midday and nightfall.

A sundial calibrated to show shaos zemanios from netz to shekiah will in fact have a minor gradual change in hour length through the day due to the fact that the year is advancing through the seasons at the same time, but this change is almost negligible in the span of one day. R’ Moshe’s shittah stems from the fact that he calculates the hours from daybreak to nightfall as explained above.

#1347990

WolfishMusings
Participant

moment of the sun’s transit

Transit of what?

The Wolf

#1348005

JJ2020
Participant

What I mean is that when they announce the molad in shul they assume that the moon moves at a constant speed. However it doesn’t. But we do it this way for convenience and simplicity.

#1348025

ubiquitin
Participant

“What I mean is that when they announce the molad in shul they assume that the moon moves at a constant speed. However it doesn’t. But we do it this way for convenience and simplicity.”

Its not just for “convenience and simplicity” the whole calendar is structured on the average speed. It wouldnt be possible to produce a calendar based on the actual molad because it varies within each cycle and cycle to cycle. EAnd forget about getting it to fit with the solar cycle , which isnt just convenient but a necessity due to chodesh haaviv.
The molad announced in shul is the one upon which our entire calendar is structured. while it isnt the “actual astronomical” molad or “molad ha’amiti” this sint because anyone is “rounding” anything

#1348036

Snagged
Participant

1. There is no such thing as ‘netz’. In the term ‘honeitz hachama’, the first word is a verb.

2. Chatzos is when is at the highest point in the sky, which is exactly halfway between sunrise and sunset.

#1348052

akuperma
Participant

You could make a kosher sun dial but it wouldn’t be as accurate. We are used to paying close attention to minutes and seconds, and a sundial might be accurate withing five or ten minutes at best. That is why they disappeared and were replaced with clocks. Not to mention that sundials never worked at night or when it was cloudy.

#1348061

ubiquitin
Participant

Snagged
“There is no such thing as ‘netz’. In the term ‘honeitz hachama’, the first word is a verb.”

I agree. however see MB 493:6, biur hagra 459:5 who use the term “neitz” (although they usually use the term “haneitz”)

#1348064

ubiquitin
Participant

Snagged
” Chatzos is when is at the highest point in the sky, which is exactly halfway between sunrise and sunset.”

the first half of your statement is true not the second.

chatzos is when the sun is at the highest point in the sky this is known as “solar noon”

See Ig”m EH Chelek 1:58 where he gives real examples for example on 4/20 Haneitz is at 5:10 while shkiah is at 6:40 the midpoint is 11:55 yet chatzos is 12:00 making the 6 morning Shaos zemanios slightly longer than the 6 afternoon shaos zemanios. This is what Halevai was reffering to in his OP

If your second assertion was true then chatzos would take place at exactly the same time every day. since it takes 24 hours for the Earth to spin on its axis. So if the sun is at its highest point at 12:00 (regardless of when sunrise/sunset take) place 24 hours later it should be back in the same place. Which while close, isnt the case. however to to the elliptical orbit and the tild effect among other factors their is some slight variability) though I dont understand R’ Moshe who seems to say Chatzos is always at 12:00

#1348087

ubiquitin
Participant

Halevai

“Why bother fixing six hours to the same length when it doesn’t reflect reality.”
the answer to your question is to get two halchos to fit namely that say sof zeman shacharis is both after 6 hours and at chatzos. Yet the reality is that these arent the same times if you diefine shaos zemanios as 1/12 of neitz to shkiyah. So R’ Moshe “redefines” shaos zemanios as 1/6th of neitz to chatzos for AM shaos zemaniyos and 1/th of chatzos to shkiyah for PM shaos zemanios.

(At least this is how I recall the Tehsuva)

#1348206

HaLeiVi
Participant

ChadGadya, what are you saying? The sundial is not quartz based. It shows the sun’s angle. The sun starts at Netz and finishes at Shkia. The space in between, when divided by twelve, would be the hours. These are obviously not the same each day, since the day itself is not the same.

#1348210

Participant

Sorry HaLeiVi, I know it may seem counterintuitive, but as I said this is a common misconception. I’ll try to explain.

Imagine an axis extending through the earth from north pole to south and extending into outer space. The sun from our perspective describes a circle around this axis once a day. The angle it makes relative to some predefined meridian is called the hour angle and it changes uniformly at the rate of 15 degrees per regular hour. The reason the length of the day changes is because in the summer more of this circle is above the horizon than below it, and vice-versa in the winter.

The simplest sundial has its gnomon (the stick whose shadow shows the time) pointing towards the north celestial pole (south in the southern hemisphere) i.e. parallel to this axis, and has its base 90 degrees from the gnomon i.e. parallel to the equator. The exact angles from horizontal and vertical will obviously depend on the latitude of the location.

On such a sundial equally spaced lines on the base 15 degrees apart extending round in a circle from the base of the gnomon will show equal regular hours of sixty minutes each. At the equinoxes the total extent of the shadow’s movement through the day will be 180 degrees, from the line marked “6 am” to the line marked “6 pm” (ignoring the equation of time (google it!) which sundials cannot take into account). In the summer the sun rises earlier and sets later, so the shadow will traverse more than 180 degrees, encompassing the lines before 6 am and after 6 pm, and in the winter the opposite, i.e. at sunrise the shadow will already be up to some line after 6 am and the sun will set before the shadow gets to six pm.

Actual sundials are more complicated than this and the marks are not equidistant. This is mainly due to the necessity of having a horizontal base, because in the winter the sun is always below the celestial equator so the sundial described above would have its shadow cast on the underside of the base (which was parallel to the equator remember). The principle however remains about the same. Making a sundial to show shaos zemanios is very complicated and involves changing the angle of the gnomon and/or the marks on the base throughout the year, and/or having a concave spherical-like base, all of which could not be done very accurately in ancient times.

#1348219

Participant

Hi Mods, Why is my previous post still awaiting moderation when I see more recent posts on other threads are already going through?

Longer posts are harder to mod while driving

#1348535

iacisrmma
Participant

Why are you moderating while driving? Isn’t it safer to wait until you arrive at your destination?

🙄

#1348558

DrYidd
Participant

to all the learned posters. shittot where chatzot does not occur at the exact astronomical point of midday have a complex halakhic history. there are at least three reasons that underlie this phenomenon. individuals like rav nosson Adler ztl, the CS ztl’s rebbe, RYCS ztl, RSZA ztl, RMF ztl maintained such a position. it was also minhag yerushalayim in the 19th century. to understand it requires a long shiur in zemanim. most poskim reject the shittah despite the many who in one way or another upheld it. even the minchat yitzchok forcefully rejects it despite its prevalence in the early yishuv.

#1348603

Participant

DrYidd, are you sure those poskim you quote aren’t only taking about midnight rather than midday? At midnight we don’t have a directly observable astronomical phenomenon as we do for midday, so it makes more sense to go for the halfway-in-hours definition there than it does for midday.

#1349695

DrYidd
Participant

ChadGadya, only RSZA ztl talks of midnight but lechol hadeyot, midday and midnight are ALWAYS 12 standard hours apart.

to all: we announce the “average lunation/ molad” the actual molad varies by as much as 9 hours, i believe, given the elliptical orbit of the earth around the sun. chazal’s estimate of the average lunation was correct to 5 decimal places. the accumulated error over 1500 years is about three hours in the right direction, making the bias in RC later. in about 5000 years a sanhedrin will have to correct deleting one 13th month and one 30th day. nothing to lose sleep over. both the 19 year cycle and the average lunation are approximations, the lunation a remarkably good one, the 19 year cycle a bit less so.

#1349715

Joseph
Participant

Of course it is an approximation. It would be virtually impractical to have set it exactly, which would result in fractions of hours/minutes that would need to be calculated each year.

Viewing 21 posts - 1 through 21 (of 21 total)
• You must be logged in to reply to this topic.