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Almagest Book III: On the Calculation of the Solar Position

This chapter is easily the shortest one in Book III. It literally consists of a paragraph (which I’ll quote in its entirely but break into two for ease of reading) that gives a very quick description of how one calculates the position of the sun at any given time from the epoch derived in the last chapter.

So whenever we want to know the sun’s position for any required time, we take the time from epoch to the given moment (reckoned with respect to the local time at Alexandria), and enter with it into the table of mean motion. We add up the degrees [and their subdivisions] corresponding to the various arguments [18-year periods, years, months, etc.], add to this the elongation [from apogee at epoch], 265;15º, subtract the complete revolutions from the total, and count the result forward from Gemini 5;30º rearwards through [i.e. in the order of] the signs. The point we come to will be the mean position of the sun.

Next we enter the same number, that is the distance from apogee to the sun’s mean position, into the table of anomaly, and take the corresponding amount in the third column. If the argument falls in the first column, that is if it is less than 180º, we subtract the [equation] from the mean position; but if the argument falls in the second column, i.e. is greater than 180º, we add it to the mean position. Thus we obtain the true or apparent [position of] the sun. Continue reading “Almagest Book III: On the Calculation of the Solar Position”

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Almagest Book III: On the Epoch of the Sun’s Mean Motion

We’ve come a long way in this book establishing a working model for solar motion. In fact, we’ve explored two models and derived a table that shows how far the sun would be away from its mean motion based on the mean position. However, at this point, everything has been done in terms of apogee and perigee.

In this chapter, we’ll be defining the “epoch“. What that means is that Ptolemy is going to pick a point in time, and define where the sun was on that date. Then, applying the tools we have developed in this book, we’ll be able to determine where the sun is at any other given date using the epoch as the starting point. For those that like things in a bit more mathy terms, it’s the location of the sun on the eccentre at time = 0, wherein Ptolemy will decide what that date is. To get there, we’ll first establish precise point on the eccentre at a known point in time, and then use the methods from this chapter to go backwards until we get to the chosen epoch date. Continue reading “Almagest Book III: On the Epoch of the Sun’s Mean Motion”

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Almagest Book III: Equation of Anomaly from Perigee using Epicyclic Hypothesis

As the final post for this chapter, we’ll examine the equation of anomaly for angles measured from perigee in the epicyclic model. Again, we’ll start with a basic diagram for the epicyclic model and this time, we’ll drop a perpendicular onto $\overline{DA}$ from H to form $\overline{HK}$.

Continue reading “Almagest Book III: Equation of Anomaly from Perigee using Epicyclic Hypothesis”

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Almagest Book III: Equation of Anomaly from Perigee using Eccentric Hypothesis

In the past couple Almagest posts, we’ve demonstrated that you can derive the equation of anomaly if you know the angle from apogee of either the mean or apparent motion using either the eccentric or epicyclic hypothesis1. Now, we’ll do the same using the angular distance from perigee again using 30º as our example. Continue reading “Almagest Book III: Equation of Anomaly from Perigee using Eccentric Hypothesis”

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Data: Stellar Quadrant Observations – 11/23/19

It’s been awhile since I’ve done a post on observing. This is in large part because I’ve been so busy with events as White Hawk herald for Their Majesties, that I’ve either been at an event, or so burnt out that I’ve been staying in. There was one small observing run between my last post and now, at Crown Tournament, but I only took 3 observations, so I didn’t feel it was worth an entire post1.

However, this month, I really wanted to get out for a long night of observing as I missed observing most of last year in November and December, so plugging that hole in my data was important to me. I ended up getting in about 3.5 hours of observing before it got too cold to want to be out anymore. Continue reading “Data: Stellar Quadrant Observations – 11/23/19”

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Almagest Book III: Equation of Anomaly from Apogee using Epicyclic Hypothesis

Despite Ptolemy demonstrating the equivalence of the eccentric and epicyclic hypotheses many times now, he sets out to prove that we can arrive at the same equation of anomaly using the epicyclic hypothesis as we do the eccentric. As we did for the eccentric, we’ll again use the case when the mean motion is 30º from apogee. Continue reading “Almagest Book III: Equation of Anomaly from Apogee using Epicyclic Hypothesis”

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Almagest Book III: On the Anomaly of the Sun – Difference Between Mean and Anomalistic Motion

In the last post, we determined the basic properties of the eccentric model to predict the motion of the sun. Now, we’ll use these properties in conjunction with the model itself to be able to predict “the greatest difference between mean and anomalistic motions” by referring back to our original model. Continue reading “Almagest Book III: On the Anomaly of the Sun – Difference Between Mean and Anomalistic Motion”

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Almagest Book III: On the Anomaly of the Sun – Basic Parameters

Now that we’ve laid out how the two hypotheses work and explored how they sometimes function similarly, it’s time to use one of them for the sun. But which one? Or do we need both?

For the sun, Ptolemy states that the sun has

a single anomaly, of such a kind that the time taken from least speed to mean shall always be greater than the time from mean speed to greatest.

In other words, the sun fits both models, but only requires one. Ptolemy chooses the eccentric model due to its simplicity.

But now it’s time to take the hypothesis from a simple toy, in which we’ve just shown the basic properties, and to start attaching hard numbers to it to make it a predictive model. To that end, Ptolemy states,

Our first task is to find the ratio of the eccentricity of the sun’s circle, that is, the ratio of which the distance between the center of the eccentre and the center of the ecliptic (located at the observer) bears to the radius of the eccentre.

Continue reading “Almagest Book III: On the Anomaly of the Sun – Basic Parameters”