[Correction (Sept 11, 2023): What I shared in the video about the difference between tropical and sidereal time is correct. But, we do not need to know the sidereal time to calculate the planet positions. For that, all we need to know is the GMT date and time. We do need to know the sidereal time to calculate house cusps. I apologize for the mistake. I talk more about this in part 8 - Correction and Calculating the Positions of the Planets.]
Notes
I use the phrase “sync up” a number of times in the video \ audio. It’s not really the best term for what I’m trying to say. What we’re doing in chart calculation is converting a given time to another kind of time. That’s different than syncing.
For the audio version of the video and for the transcription of both, scroll down to the last section.
Finally, please ask questions if something doesn’t make sense. I’ll do my best to answer them.
Time
When you learn to calculate charts by hand, you encounter the strange phenomenon of time. Specifically, you discover that there are different kinds of time. If you follow that track a little down the line, you learn about the different motions of the Earth and how they create the different kinds of time. And so you start to learn a little about the home we all live on while we’re here in human form, and the elegant complexity that goes into making this realm, and thus our experience, possible.
The graphic at the end of this section illustrates the difference between the tropical and sidereal days. I share it so that you have a visual of the difference between these two measures of time. In the video, I explain a little more about what the difference is. But to calculate charts, it is enough to know that there are two different measurements of time we are working with and that the reason we are doing the calculations is to convert one type of time to the other.
The first kind of time is called tropical time. As I explain in the video, that is the time we use to organize, plan, and record our lives. It is also called solar time. The second type of time relevant to chart calculation is sidereal time. This kind of time is what astronomers use to record the positions of the planets in the sky.
When we calculate charts, we want to know where the planets are at the given event time and location. Because of the discrepancy between the measure of time used to record events and that used to record planet positions, we have to convert the given time to sidereal time.
As stated, the graphic below provides a visual of the discrepancy between a solar (or tropical) day and a sidereal day. In this course, we just want to know that there is a difference. We won’t be going into the details of why there is a difference. Just note that there is one.
[Update 7/19/2023: Here is the image I was referencing. It seems I forgot to include it, though I swore I did!]
Next Up
Next up we’ll look at time zones and do the basic time zone conversions. These conversions are the first steps to getting an accurate planetary time for an event. We’ll also take a look at multiplying and dividing degrees, minutes, and seconds in preparation for one of the next steps in converting the event time to sidereal time.
Below is the audio version of the video with an edited transcription.
Audio Version
Edited Transcription
The other why is, Why do we even have to do it [chart calculations]. Again, if someone comes and gives us a chart, why can’t we just draw up a chart? Why do we have to go through all this process of calculating, you know, adjusting the time so that it syncs up with something. What does it sync up with? Anyway, so that's what I’m going to talk about.
So the main reason is because there are two kinds of time relevant to astrology in terms of chart calculation. One is called tropical time and the other is called sidereal time. Tropical time we can say is human time in the sense that it's a time we use to record our our lives, our events. It’s what we use to plan our lives. Tropical time is seasonal time and it's calendar time. Calendar time is seasonal time… Sidereal time is a time we use… to record, or the astronomers use to record the positions of the planets in the sky. So it's a different time system. And it so happens that these two times are different from one another.
So tropical time is, like I said, seasonal time. So it's about the… Earth’s relationship with the Sun. So as the Earth travels around the Sun… we experience different seasons because of the relationship between the Sun and the Earth. So we know about the solstices and the equinoxes. They start our seasons. They’re marking specific points in the… Earth’s relationship with the Sun. And how we calculate a tropical year, is we take one of those points, we’ll say the vernal equinox because that’s the point that make most sense for astrology. We take the vernal equinox and we mark that spot. We say, okay, this is where the relationship alignment is between the Sun and the Earth at this moment. And so we mark that spot and then we just wait and watch and as - what it looks like to us - as the Sun comes back around to this spot. When it reaches that spot, we mark that and that’s a tropical year.
Tropical comes from the Greek word tropikos and tropikos means change or turn.1 And so what that’s referencing is that the solstices - at the Summer solstice, the Sun the Sun reaches its highest point in the sky as it appears to us. And at that point, it changes direction. It turns. It turns the direction it was moving in. It changes and starts moving downward, starts going lower in the sky. And then the Winter solstice, it's the same thing only different. The Sun reaches its lowest point. And then at that moment, it changes its direction. It turns. It starts turning up and going higher in the sky. And so that’s where the name tropical comes from.
Those points, the highest and lowest points, are marked by the Tropics of Cancer and Capricorn. And so, those are the imaginary latitude lines that we use to help us understand where things are on the earth. And then they also let us know, they also happen to mark… I don’t know if it’s happened. They probably decided that on purpose. I actually don't know that. But they… mark those highest and lowest points of the Sun’s journey as it looks from our perspective. So that's the tropical year. And a tropical day is also the Earth’s relationship with the Sun. And at this point we’re just marking… We’re using the rotation of the Earth rather than the orbit of the Earth to determine… a tropical or solar day. So tropical can also be said solar - solar year and solar day.
And so then there’s the sidereal year and sidereal means relating to the stars and that is, again, that's about the Earth’s relationship with the stars as opposed to with the Sun. So, as the Earth orbits the Sun, along that orbit or sort of in the background of that orbit, lining the orbit are the constellations - the zodiac belt of constellations. So it's like the Earth is traveling through the zodiac belt of constellations… From our perspective, of course, it looks like the Sun is moving around the Earth and so it looks like the Sun is moving through that zodiac belt of constellations. So what we can do is we can look out and pick a spot. Where is the Sun right now and… what point in the constellation, the zodiac belt of constellations is it in… We mark that spot. And then, like we did before, we watch and wait. And when the Sun, again, appears to come back around to that point in the constellation, that marks a sidereal year.
And it so happens that a sidereal year is twenty minutes longer than a tropical year. And that has to do with… a motion of the Earth called precession. But I won’t say more about that. Just know that that's why that happens… And then a sidereal [day - I said year in the video \ audio] is just the Earth's rotation on its axis again… But the reference point this time is the constellations rather than the Sun. And the sidereal day is actually shorter, four minutes shorter, than a tropical day.
And so, no matter how you look at it… tropical time and sidereal time don't sync up. And so we have to make calculations to get them to sync up… Because… we record events in tropical time and… planet positions are recorded in sidereal time, we have to make calculations to get the… event time to sync up with the sidereal time.
References
tropic (n.). (n.d.). In Online Etymology Dictionary / etymonline.com. Retrieved July 14, 2023, from https://www.etymonline.com/word/tropic
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