<p><strong>Edit</strong> <em>(massive rewrite in response to comments)</em></p>

<p>There's nothing "out there" that will tell you what day of the week it is.
The concept of a week is determined by religion or culture but there's no objective reason why a week should have seven days.  From 1793-1805 the French Republic had a calendar with a <a href="http://en.wikipedia.org/wiki/French_Republican_Calendar#Ten_days_of_the_week" rel="nofollow">10-day week</a>.</p>

<p>If you wanted to try to extrapolate our idea of a week to other planets, be aware that the planets in our solar system have very different <a href="http://www.bobthealien.co.uk/table.htm" rel="nofollow">day lengths</a>.  One Venus day is 243 Earth days, while the planets Jupiter through Neptune all have days that are less than one Earth day.</p>

<p>It doesn't happen in our own solar system, but it's theoretically possible for a planet to have one side always facing its star.  Then it would be impossible to define a day in the way that you've intended.</p>

<p>Even if you wanted to know what day of the year it is, that's possible but quite hard.  In particular you would have to account for things like time zones, leap years, and daylight savings, which are not empirically observable phenomena.  A few hundred years ago the definition of noon was when the sun was at its highest point in the sky.  I think that definition is closer to what you're asking, but that's not the calendar system that we currently use.</p>

CC BY-SA 4.0
2021-05-21T09:11:12.080Z
The lunar period has 28 days, this gives the notion of month (although as we all know, it has suffered many modifications and today there is only one month which has 28 days). It was natural to divide this period of 28 days into more manageable periods, and so arose the week which is 1/4 of a lunar period. So it's not entirely correct to say that there's no objective reason to have a seven day week.
Leo
CC BY-SA 3.0
2014-07-04T22:36:47.020Z
Although this is interesting, it doesn't directly answer the question.
LDC3
CC BY-SA 3.0
2014-07-05T20:23:42.403Z
More specifically, it answers the 'related note' only. Improve the quality of the answer by also addressing the primary question.
Jeremy
CC BY-SA 3.0
2014-07-05T21:12:44.417Z
Yeah I'm new here... trying again.
Matthew Jacobs
CC BY-SA 3.0
2014-07-04T20:26:07.343Z
2014-07-05T21:10:01.067Z
2014-07-05T21:10:01.067Z
Matthew Jacobs
<p>It is possible to determine the day of the year using the distance of the sun from the vernal equinox (you have to account for the equation of time etc, but it is possible). It is also possible to determine the year if your measurements are precise enough, using the current position of the vernal equinox (which changes due to precession). The ambiguity of the factor of 26000 years can be removed by noting the positions of other stars (for example, Barnard's star). Knowing these two, you should be able to find the day of the week.</p>

<p>About your second question, Astronomers like to deal with Julian date, which is a continuous day counting system, and you can determine the day of the week from the Julian date, though only if given a reference, for example Jan 1, 2000, was a Saturday. As such, a week and days of a week are man made (<a href="https://en.wikipedia.org/wiki/Week">https://en.wikipedia.org/wiki/Week</a>).</p>

CC BY-SA 3.0
2014-07-04T11:45:43.370Z
Also, for the year, you have to account for nutation. I forgot to add that.
Takku
CC BY-SA 3.0
2014-07-04T17:39:17.377Z
Ok, the first part is clear now, thank you, but I still don't get the second one. Perhaps I wasn't clear in the question. I accept that the days of the week are man-made, but that doesn't mean that they can't be absolute. For instance, the metre is an ad-hoc man-made unit of measurement, but today we define it as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second”. I wonder if there exists a similar definition for, say, Sunday.
questionguy
CC BY-SA 3.0
2014-07-04T17:40:13.837Z
By the way, I just realized that the Gregorian reformation didn't affect the days of the week at all, so we can use the Julian calendar as well.
questionguy
CC BY-SA 3.0
2014-07-04T19:01:53.727Z
About the absoluteness, you only need to define one day (Jan 1, 2000 you can say is defined as being a Saturday). This makes all other days absolutely defined. And afaik, there is no physical/astronomical definition for it apart from an arbitrary definition/assignment.
Takku
CC BY-SA 3.0
2014-07-04T19:48:09.957Z
I see. I hoped for a bit more, but it seems like I'll have to face the reality. :)
questionguy
CC BY-SA 3.0
2014-07-04T20:11:00.110Z
In that case, the 'afaik' part should help. You (or someone else who's reading this) could research a bit more as to whether there actually is a real origin for at least some particular day being a particular day of the week. This is how you find unanswered questions and try to solve them as part of a research career. I would love to know if there actually is a specific origin or it's plain arbitrary. My knowledge is my limit (currently), not yours.
Takku
CC BY-SA 3.0
2014-07-04T22:42:01.900Z
The internationally accepted week system has cultural and religious origins.  Other than the fact that a 'day' is defined by an astronomical event, the week or its individual days have no significant relationship with astronomy.  For this reason, you could calculate it using an index point of a day with a known day of the week as Takku has already described, but not without.
Mitch Goshorn
CC BY-SA 3.0
2014-07-05T02:00:20.817Z
Ok, it's not the index point that really bothers me. I understand that it's a cultural and religious issue, but that only affects shifting and there still remains an astronomical question beyond that. Let me propose another thought experiment to make it clear. I'll drop you off at several planets and each time you should tell me which day of the week it is. You can say whatever you want, but if I take you back to the same planet at some later time, you'll have to be consistent (e.g. if you said Wednesday first and I bring you back 100 “days” later, you should say Friday now). Can you do that?
questionguy
CC BY-SA 3.0
2014-07-05T03:30:38.447Z
Given no other information, no. The method described relies heavily on knowledge of local bodies from which to derive the number of days since a point of index. This is very easy on Earth due to the abundance of information available to us.  On a hypothetical planet in which this information isn't available, we have no way to derive days passed.
Mitch Goshorn
CC BY-SA 3.0
2014-07-05T07:46:24.267Z
I agree with Mitch. We have defined references to an exact earth day & an exact earth year and we know about the precession & nutation periods in terms of earth years, from which we can get the date. Also, weeks are defined for earth days, so if we do go to other planets, we'll still have to work with earth days. We don't know the constants as accurately for other planets (and definitely not for hypothetical planets, but I don't think we're dealing with those here). And it is nearly impossible to calculate such relations in a time frame as small as a couple of days as was mentioned in the OP.
Takku
CC BY-SA 3.0
2014-07-08T20:50:37.987Z
I've been thinking a bit, and yes, you're right. Suppose we have a star far enough from the others with a single planet orbiting around it and no other spatial bodies nearby. Then the planet has a perfect elliptical orbit. Now assume that a "year" consists of precisely 100 "days". If I drop you off on the planet twice with a difference of a "year", you'll see exactly the same (as the entire system is in the same state), but you need to give a different answer. For a deterministic method, that's a contradiction.
questionguy
CC BY-SA 3.0
2014-07-04T05:46:15.517Z
2014-07-04T10:17:20.397Z
2014-07-04T10:17:20.397Z
Takku
<p>Suppose I wake up from a coma on a desert island in the 19th century (i.e. we already use the Gregorian calendar but have no satellites yet). I have a clear view of the sky and a couple of days to make observations and calculations. Can I determine the current day of the week? Also, can I find the answer instantly?</p>

<p>On a related note, is it possible to define days of the week in a purely astronomical (e.g. planet-agnostic) manner such that for the Earth we get the traditional meaning?</p>

CC BY-SA 3.0
2016-01-28T23:53:05.543Z
Do we know how long the coma lasted, and the weekday of the last full moon before you entered the coma? If so, the 28 day lunar cycle should get you the day of the week.
Wayfaring Stranger
<earth><amateur-observing><observational-astronomy>
Can I look at the sky and find the day of the week?
questionguy
# Can I look at the sky and find the day of the week?

(Asked by: username_0 on 2014-07-04)

Suppose I wake up from a coma on a desert island in the 19th century (i.e. we already use the Gregorian calendar but have no satellites yet). I have a clear view of the sky and a couple of days to make observations and calculations. Can I determine the current day of the week? Also, can I find the answer instantly?

On a related note, is it possible to define days of the week in a purely astronomical (e.g. planet-agnostic) manner such that for the Earth we get the traditional meaning?

**Tags:** earth, amateur-observing, observational-astronomy

> **Comments:**
> **username_1**: Do we know how long the coma lasted, and the weekday of the last full moon before you entered the coma? If so, the 28 day lunar cycle should get you the day of the week.  

---

## 2 answers

### Answer by username_6 on 2014-07-04. Score: 6 | Accepted answer

It is possible to determine the day of the year using the distance of the sun from the vernal equinox (you have to account for the equation of time etc, but it is possible). It is also possible to determine the year if your measurements are precise enough, using the current position of the vernal equinox (which changes due to precession). The ambiguity of the factor of 26000 years can be removed by noting the positions of other stars (for example, Barnard's star). Knowing these two, you should be able to find the day of the week.

About your second question, Astronomers like to deal with Julian date, which is a continuous day counting system, and you can determine the day of the week from the Julian date, though only if given a reference, for example Jan 1, 2000, was a Saturday. As such, a week and days of a week are man made (<https://en.wikipedia.org/wiki/Week>).

> **Comments:**
> **username_6**: Also, for the year, you have to account for nutation. I forgot to add that.  
> **username_0**: Ok, the first part is clear now, thank you, but I still don't get the second one. Perhaps I wasn't clear in the question. I accept that the days of the week are man-made, but that doesn't mean that they can't be absolute. For instance, the metre is an ad-hoc man-made unit of measurement, but today we define it as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second”. I wonder if there exists a similar definition for, say, Sunday.  
> **username_0**: By the way, I just realized that the Gregorian reformation didn't affect the days of the week at all, so we can use the Julian calendar as well.  
> **username_6**: About the absoluteness, you only need to define one day (Jan 1, 2000 you can say is defined as being a Saturday). This makes all other days absolutely defined. And afaik, there is no physical/astronomical definition for it apart from an arbitrary definition/assignment.  
> **username_0**: I see. I hoped for a bit more, but it seems like I'll have to face the reality. :)  
> **username_6**: In that case, the 'afaik' part should help. You (or someone else who's reading this) could research a bit more as to whether there actually is a real origin for at least some particular day being a particular day of the week. This is how you find unanswered questions and try to solve them as part of a research career. I would love to know if there actually is a specific origin or it's plain arbitrary. My knowledge is my limit (currently), not yours.  
> **username_7**: The internationally accepted week system has cultural and religious origins.  Other than the fact that a 'day' is defined by an astronomical event, the week or its individual days have no significant relationship with astronomy.  For this reason, you could calculate it using an index point of a day with a known day of the week as username_6 has already described, but not without.  
> **username_0**: Ok, it's not the index point that really bothers me. I understand that it's a cultural and religious issue, but that only affects shifting and there still remains an astronomical question beyond that. Let me propose another thought experiment to make it clear. I'll drop you off at several planets and each time you should tell me which day of the week it is. You can say whatever you want, but if I take you back to the same planet at some later time, you'll have to be consistent (e.g. if you said Wednesday first and I bring you back 100 “days” later, you should say Friday now). Can you do that?  
> **username_7**: Given no other information, no. The method described relies heavily on knowledge of local bodies from which to derive the number of days since a point of index. This is very easy on Earth due to the abundance of information available to us.  On a hypothetical planet in which this information isn't available, we have no way to derive days passed.  
> **username_6**: I agree with username_7. We have defined references to an exact earth day & an exact earth year and we know about the precession & nutation periods in terms of earth years, from which we can get the date. Also, weeks are defined for earth days, so if we do go to other planets, we'll still have to work with earth days. We don't know the constants as accurately for other planets (and definitely not for hypothetical planets, but I don't think we're dealing with those here). And it is nearly impossible to calculate such relations in a time frame as small as a couple of days as was mentioned in the OP.  
> **username_0**: I've been thinking a bit, and yes, you're right. Suppose we have a star far enough from the others with a single planet orbiting around it and no other spatial bodies nearby. Then the planet has a perfect elliptical orbit. Now assume that a "year" consists of precisely 100 "days". If I drop you off on the planet twice with a difference of a "year", you'll see exactly the same (as the entire system is in the same state), but you need to give a different answer. For a deterministic method, that's a contradiction.  

---

### Answer by username_2 on 2014-07-04. Score: 4

**Edit** *(massive rewrite in response to comments)*

There's nothing "out there" that will tell you what day of the week it is.
The concept of a week is determined by religion or culture but there's no objective reason why a week should have seven days. From 1793-1805 the French Republic had a calendar with a [10-day week](http://en.wikipedia.org/wiki/French_Republican_Calendar#Ten_days_of_the_week).

If you wanted to try to extrapolate our idea of a week to other planets, be aware that the planets in our solar system have very different [day lengths](http://www.bobthealien.co.uk/table.htm). One Venus day is 243 Earth days, while the planets Jupiter through Neptune all have days that are less than one Earth day.

It doesn't happen in our own solar system, but it's theoretically possible for a planet to have one side always facing its star. Then it would be impossible to define a day in the way that you've intended.

Even if you wanted to know what day of the year it is, that's possible but quite hard. In particular you would have to account for things like time zones, leap years, and daylight savings, which are not empirically observable phenomena. A few hundred years ago the definition of noon was when the sun was at its highest point in the sky. I think that definition is closer to what you're asking, but that's not the calendar system that we currently use.

> **Comments:**
> **username_4**: Although this is interesting, it doesn't directly answer the question.  
> **username_5**: More specifically, it answers the 'related note' only. Improve the quality of the answer by also addressing the primary question.  
> **username_2**: Yeah I'm new here... trying again.  
> **username_3**: The lunar period has 28 days, this gives the notion of month (although as we all know, it has suffered many modifications and today there is only one month which has 28 days). It was natural to divide this period of 28 days into more manageable periods, and so arose the week which is 1/4 of a lunar period. So it's not entirely correct to say that there's no objective reason to have a seven day week.  

---

