Why Does the Analemma Show an Ellipse for Solstices?

  • Context: Undergrad 
  • Thread starter Thread starter JeffOCA
  • Start date Start date
  • Tags Tags
    Time
Click For Summary
SUMMARY

The discussion centers on the representation of solstices in an analemma, which is a figure 8 shape that marks the position of the sun at true solar noon throughout the year. Participants clarify that solstices are represented by an ellipse rather than a single point due to the variability of solstice dates (June 20, 21, or 22 and December 20, 21, or 22) and the low declination speed of the sun near these dates. This results in multiple potential positions being too close to distinguish individually, necessitating the use of an ellipse as an envelope for these positions.

PREREQUISITES
  • Understanding of solar declination and its impact on solar positioning.
  • Familiarity with the concept of analemma and its geometric representation.
  • Knowledge of the Gregorian calendar and the variability of solstice dates.
  • Basic principles of astronomy related to the Earth's orbit and axial tilt.
NEXT STEPS
  • Research the mathematical derivation of the analemma shape and its significance in astronomy.
  • Explore the concept of solar declination and its calculation throughout the year.
  • Investigate the historical significance of analemmas in navigation and timekeeping.
  • Learn about the construction and use of permanent analemmas in public spaces.
USEFUL FOR

Astronomy enthusiasts, educators, and students studying celestial mechanics, as well as anyone interested in the practical applications of solar positioning in navigation and timekeeping.

JeffOCA
Messages
48
Reaction score
0
Hi,

In a regular analemma, you have 365 marks on the ground where the sunlight fall every day at true solar noon. The equinoxes and solstices are also represented on the analemma.

To indicate the solstices, you don't have a point on the ground but a kind of ellipse. I think this is due to solstices that don't strictly occur the same day (20, 21 or 22 june / december) and also because near solstices the declination speed is very low, so the positions are too close to be represented individually, so an ellipse as an envelope of the different possible positions.

Can you confirm or infirm ?

Thanks
 
Astronomy news on Phys.org
What do you mean by a "regular analemma"?

As far as I know, an analemma IS an ellipse. If you mark a spot each week or day for an entire year and then connect the dots you will make a figure 8 ellipse that shows the movement of the Sun in the sky for that year.

http://en.wikipedia.org/wiki/Analemma
 
I know analemma makes a figure 8 slightly deformed. The word "regular" wasn't appropriate, forget it !
My question was (sorry if it was not clear enough) : each mark on the ground is a point for each day of the year but to mark the solstices we have a kind of ellipse containing several points. Why we have such an ellipse to mark solstices ?

I hope it's more clear now.

Thanks
 
Anyone ?
 
I have no idea what you are talking about. What ellipse are you referring to? The analemma makes a figure 8, so I don't see any ellipses other than the two ellipses that form the figure 8.
 
Drakkith said:
I have no idea what you are talking about. What ellipse are you referring to? The analemma makes a figure 8, so I don't see any ellipses other than the two ellipses that form the figure 8.

Sorry for the misunderstanding, but english is not my mother tongue. I agree analemma makes a figuer 8. On this figure 8, you have on the floor 365 marks for the 365 days of the year. You have also other indications like months, equinoxes, solstices, etc.
But, on the figure 8, the position of the solstice is not marked by a single dot but with a little ellipse (in dotted line). Why ?

My first ideas were :
- Solstice doesn't strictly occur at the same day (20, 21 or 22 june / december) for a given year.
- in addition, near solstices the sun declination speed is very low, so the positions of the sunspot on the ground are too close day by day to be represented individually, as singles positions. So we draw an ellipse which the "envelope" of the different possible positions for solstice.

I hope that my words are clearer now...

Thanks
 
Are you talking about an analemma for a single year, or multiple years?
 
Drakkith said:
Are you talking about an analemma for a single year, or multiple years?

This analemma (where I see an ellipse for delimiting the solstice) is engraved in the ground, so i think it is an permanent analemma, for multiple years...
 
JeffOCA said:
This analemma (where I see an ellipse for delimiting the solstice) is engraved in the ground, so i think it is an permanent analemma, for multiple years...

I'm not sure, but I think your reasoning is correct in this case. Since the solstice occurs at different times each year it would not be a single spot.