Analemma and equation of time

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In summary: The ellipse is used to indicate the solstice since the positions would be too close together to be represented individually.
  • #1
JeffOCA
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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
 
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  • #2
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
 
  • #3
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
 
  • #4
Anyone ?
 
  • #5
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.
 
  • #6
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
 
  • #7
Are you talking about an analemma for a single year, or multiple years?
 
  • #8
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...
 
  • #9
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.
 

1. What is an analemma and how does it relate to the equation of time?

The analemma is a figure-eight shaped curve that represents the apparent motion of the Sun in the sky over the course of a year. It is caused by the Earth's tilt on its axis and its elliptical orbit around the Sun. The equation of time is a mathematical expression that corrects for the difference between the apparent solar time and the mean solar time, which is based on a uniform 24-hour day. The analemma and equation of time are closely related because the equation of time is used to determine the shape and position of the analemma.

2. Why does the equation of time change throughout the year?

The equation of time changes throughout the year because the Earth's orbit around the Sun is not perfectly circular. As a result, the Earth's speed varies as it moves around the Sun, causing a difference between the apparent solar time and the mean solar time. This difference is also affected by the tilt of the Earth's axis, which changes the angle at which the Sun's rays hit the Earth's surface.

3. How does the analemma affect the accuracy of sundials?

Sundials use the position of the Sun in the sky to measure time, but the apparent position of the Sun is affected by the analemma. This means that sundials are not always accurate and may need to be adjusted throughout the year to account for the changing position of the Sun. However, modern sundials can be designed to compensate for the analemma, resulting in more accurate timekeeping.

4. What is the significance of the equation of time in astronomy?

The equation of time is significant in astronomy because it allows astronomers to accurately calculate the position of the Sun in the sky. This is important for determining the location of celestial objects and for making precise measurements of time and space. The equation of time is also used in the construction of accurate calendars and in navigation.

5. How can the analemma and equation of time be visualized?

The analemma and equation of time can be visualized through a variety of methods, including diagrams, graphs, and computer simulations. These visualizations show the changing position of the Sun in the sky throughout the year and the difference between apparent solar time and mean solar time. They can also be observed in person by tracking the position of the Sun at the same time each day for a year and marking the points on the ground where the Sun's shadow falls.

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