Overview of the Chart Calculation Process & First Steps

Hi Everyone,

This is part 4 of the chart calculation course. It’s longer than anticipated. I wanted to give an overview of the process and also didn’t want to delay getting into the first steps of calculating the chart. Including them together made the article longer than I expected it to be. I also wanted to make sure I explained things thoroughly and, in some cases, repeated information because I thought it would help with assimilation. It helped me ;). All of that added length, but I think it works out ok. Feedback is welcome.

On the video front, I go over time zones, which is relevant to the time calculations we make in this lesson. But I cover it from a different perspective and it is meant to complement the material in the article. I am planning to do more with it in the next article.

Also, I made a mistake in the video. I said that New York is at 75° W longitude. It’s actually at 74°. You have to go about 56 miles to the west to get to 75°. But the idea I was trying to convey remains the same. Finally, with regard to the connection between the Sun, time, and longitude, I am drawing on what I learned from the first article in Ken Ward’s series on chart calculation, Astrology: Calculating the Chart - UT or GMT.

If anything in the video or article isn’t clear, please feel free to ask questions. I’ll do my best to answer them.

The articles to come should be shorter.

Introduction

In this lesson, we’ll go over what’s involved in calculating charts and then walk through the first few steps. As I went over in part 2 of the introduction, the reason we need to calculate astrology charts is because the tropical time we use to plan and record our lives is different than the sidereal time used to track the movements and record the positions of the planets in the sky. Since we want to know where the planets were on a given date and time, we need to convert the event date and time to the sidereal date and time. To make this conversion, we work with three basic types of data, which I go over below.

[Correction (Sept 11, 2023): I was mistaken about needing to know the sidereal time to calculate planet positions. We don’t. We just need to know the GMT date and time to do that. But, we do need to know the sidereal time to calculate the house cusps. I apologize for the mistake. I talk more about this in part 8 - Correction and Calculating the Positions of the Planets.]

Three Types of Data

Given Data

This is the data for the event, including date, time, and location. For birth charts, we are usually given this data. But for historical and other events, and even for birth data, finding it may involve research. So that we can tackle the basics of how to calculate a chart, for this course, we’ll work with data that is given to us.

One thing to note here is that, for birth data, the best source is an official birth record. Wisdom passed down in astrology is that birth times that come from memory are unreliable.

Found Data

Found data is the data, or information, we need, in addition to the given data, to be able to calculate the chart. Traditionally, we find this information in various resources compiled by others. I talked about these resources in the previous lesson, which I’ll link to below. These resources include the following:

• An Atlas (of time zones and latitudes and longitudes)

• An Ephemeris (of planet positions and corresponding sidereal time)

• A Table of Houses (with lists of house cusps organized by time and latitude)

Today, we can find a lot of this data online. In the previous lesson on resources, I shared some online alternatives in addition to books. I share an additional online source in this article - an alternative I found for latitudes and longitudes. Just know that many online resources with the data we need are restricted to personal use only, so check the terms to see what’s allowed if you want to use the information for more than that.

Here is the link to the previous lesson:

Resources \ Introduction to Calculating Charts by Hand

3. Calculated Data

This is the data we calculate from both the given and found data. It involves three basic parts, as follows:

Part 1: Time Calculations

After gathering the data, the first part of the calculation process is to convert the given event date and time to the sidereal date and time equivalent. As explained, we need to make this conversion because the event date and time we start with is given to us in one time system (tropical) but planet positions are recorded using another (sidereal). Because we want to know where the planets were in the sky when our event happened (or happens or will happen), we need to know the sidereal date and time of the event.

This step involves a series of intermediary steps. We’ll go over the first two in this article and then continue in the next article.

Part 2: Planet Position Calculations

In this step, we determine where the planets were (or are or will be) for the event we are considering. Having calculated the local sidereal date and time in part 1, we can now calculate the planet positions. Because the ephemerides we use list planet positions for either midnight or noon for a given day and our event time is likely different than either of those times, we need to adjust the listed planet positions to where they were, are, or will be when the event occurs.

By the end of this step, we will have what we need to draw up a chart on the natural wheel. While we won’t be able to analyze the fields of energy shown in the chart for a given location (because we need the houses for that), we can get a general overview of the planetary dynamics. This can be helpful when looking at lunations and other planetary event charts.

But, to individualize a chart to a specific person or event, we need to calculate the house cusps. I explain what those are next.

Part 3: House Cusp Calculations

The houses are a complex subject that I have not delved into beyond getting an overview of the topic. I hope to dive into it at some point and share what I learn. I list some sources in the Notes section at the end for those who might be interested in exploring it.¹

For our purposes, here’s what we need to know. In astrology, we divide the chart into 12 slices. Each slice represents an area of life. We call these slices houses. The line that indicates the start of each house is called a cusp. Thus, to know where a house begins, we need to know where the cusps are. That requires some calculations.

Symbolically, each house signifies an aspect of ourselves and / or our life. The first house symbolizes identity and personality. The second symbolizes resources and worth. The third house symbolizes siblings and our immediate environment, etc. When we calculate the house cusps for a specific time and location, we open up access to more individualized symbolic insight into the chart.

To be able to do the calculations, we need the data from a source with tables of houses. These tables list house cusp positions by time and latitude. We use these lists to zero in on the house cusp positions for a given chart. Because these are not listed for every exact time and every exact latitude, we need to adjust what we find in the tables to determine the precise cusps for a given time and location.

Onto the First Steps!

In this lesson, we’ll cover the first three steps of the chart calculation process. These are:

1. Choosing the event (the given data)

2. Gathering the data we need (the found data)

3. Doing our first set of calculations²³

One thing to note here is that, for this run-through, we’ll be focusing on just one chart. It will be for an event located in the Northern Hemisphere and west of Greenwich. After we work through all the calculations for this chart, we’ll work through a chart for an event that is east of Greenwich and then a chart for an event in a southern location. Or perhaps we’ll use the same event but locate it in different places. Whatever we end up doing, we’ll need to tweak the time calculations for them to work for the different locational circumstances.

Choosing Our Event (The Given Data)

For our event, we'll use the Virgo New Moon coming up in September of this year.⁴ To get started, we can find the basic event details on the website for the Griffith Observatory. The Griffith Observatory is a public resource in Los Angeles, California. On their site, they have a table with the dates and times for the New, Full, and Quarter Moons for 2023. Referring to their table, we see that the Virgo New Moon will occur on September 14, 2023, at 18:40 (6:40 PM). Because the observatory is in Los Angeles, our location is Los Angeles. The time zone in effect is also listed, but, for now, we just want to collect the event date, time, and location:

Virgo New Moon (Given Data)

• September 14, 2023

• 18:40 (6:40 PM)

• Los Angeles, California, USA

Going forward, we’ll use 24-hour time because it is easier to do and make sense of calculations when working with time written that way.

Gathering the Additional Data (Found Data)

As explained above, we can’t use the event date and time we are given to calculate the chart. Instead, we need to convert each to its sidereal equivalent. To do that, we need some additional information, including:

• time zone information

• latitude and longitude

• sidereal time

To find the initial sidereal time, we first need to adjust the event time to Greenwich Mean Time (GMT), so we’ll hold off on gathering it until we’ve completed the GMT adjustment. But, in the meantime, we can gather the latitude, longitude, and time zone information. If you have The American Atlas on hand, you can use that to gather the data. If you don't have it, it might be best to wait before buying the book, in part because you may decide that using online astrology programs available for free will fit your needs or that you would rather invest in an astrology software program to do the work for you. Another reason to consider might be that if, like me, you’d like to create a database of some sort, you are looking for resources that aren’t restricted regarding collecting the information in this way.

If you do have a copy of The American Atlas already but haven’t used it, there is a page at the beginning of the book that explains what all the data in the tables means. That information will set you up to be able to make use of the book.

For this course, I will refer to and draw on online resources.

Time Zone

The Moon phase table on the Griffith Observatory’s site gives us the time zone that will be in effect for the Virgo New Moon. It is PDT or Pacific Daylight Time. But this information won’t always be given to us. If it isn’t, we can go to our resource of choice to find it. When we go there, we want to look for three pieces of information:

1. the time zone in effect (either daylight savings time or standard time)

2. the standard time zone for the location (if different from the time zone in effect)

3. the time zone offsets (for the time zone in effect and standard time, if different)

When using The American Atlas to gather this information, there are two caveats. The first is that it doesn’t give the time zone offsets, or how many hours different the event time is from Greenwich Mean Time or Universal Time, so you have to use it with another resource. The second is that it is protected by copyright so comes with the same limitations as some of the online resources.

Regarding the first caveat - finding the time zone in The American Atlas - we can use an online resource to use with it to get the offsets. Wikipedia has an entry that lists all the time zone abbreviations with their offsets called List of time zone abbreviations. I don’t know how accurate it is when it comes to older dates, but take it on faith that it is accurate for recent dates.

We can bypass the time zone offset issue altogether for events occurring within the current year by using an online resource with all the information. I like Time.is. As with the Wikipedia list of abbreviations, I assume the data provided by Time.is is accurate, but am still in the early stages of using it. There are other sources out there, but I like this one so I’ll go over how to use it to get the time zone information for the Virgo New Moon chart. But feel free to find and use the resource or resources that work best for you.

Navigating to Time.is, we see that one of the major cities listed on the homepage is Los Angeles. We can click on that to pull up the time zone information. (If the city or town for an event location is not listed on the home page, you can use the search bar in the upper right corner of the page, next to the menu icon, to search for it.)

Once Los Angeles is selected, scroll down if necessary to see the section that shows the dates when daylight savings time began and when it will end for this year. You’ll see that those dates are March 12th and November 5th, respectively. Since the Virgo New Moon is in September, we see that it will occur when daylight savings time is in effect.

It also tells us that the daylight savings time zone for Los Angeles is Pacific Daylight Time (PDT) and that PDT is 7 hours behind (-7) UTC. For our purposes, we can think of UTC (Universal Time Coordinated) as Greenwich Mean Time (GMT). In addition to these two terms, there is also Universal Time (UT) and Ephemeris Time (ET). Though they are not all exactly the same, they are all some version of a standardized time. We don’t have to be picky about which term we use. I will often use GMT as a generic term, even though the data I’m using might actually come from one of the other variations. I have seen ephemerides, for example, use UT or ET, but I may use the term GMT because it is more familiar and conveys the generic meaning I want to convey.

Returning to Time.is, we can write down the daylight savings time zone information it gives us, including the offset:

• Time zone in effect: PDT (-7)

Time.is also tells us that the standard time zone for Los Angeles is Pacific Standard Time (PST) and that PST is 8 hours behind (-8) UTC. So we can write that down:

• Standard time for Los Angeles: PST (-8)

We can also note the difference between the daylight savings time offset and the standard time offset. We’ll call this difference the Local DST Offset. Seeing that the standard time offset from UTC is -8 hrs and the DST offset is -7 hours, we recognize that the difference between them is 1 hr. We also note that to get to -7 from -8, we have to add an hour. That makes sense. For daylight savings, the clocks are always turned ahead, so our offset will be positive. To confirm: if the offset for standard time is -8 hours and we add one hour for the daylight savings time change, we get -7 hours. Thus, we have:

• Local DST Offset: 1 hr (+)

We’ll make use of this information below. For now, we’ll move on to gather the latitude and longitude information.

Latitude and Longitude

We can now look for the latitude and longitude for Los Angeles. This information is also found in The American Atlas, but we can find it online. Although I recommended the US Naval Academy’s Find Latitude and Longitude web application in the previous article on resources and still like it, I’ll be using a new resource I found, the USGS Earth Explorer, as it better fits my need for freely available data.⁵

Unlike the US Naval Academy’s tool, which gives the latitude and longitude in both degrees, minutes, and seconds (00° 00′ 00″), and decimal degrees (0.00°), the Earth Explorer gives the latitude and longitude in decimal degrees only. That’s ok. If we need to, we can determine the degrees, minutes, and seconds from the decimal degrees using a calculator or doing some math by hand.

For the sake of thoroughness, I like to record both versions during the gathering phase of the process, so I’ll do that below. Because I don’t want to overwhelm you with too much information, I’ll go over the math in the next article. But we can use a degree calculator to make the conversions. There are plenty freely available online. I like this one on the Federal Communications Commission’s site. On the page, there are two calculators, one to convert degrees, minutes, and seconds to decimal degrees and another, beneath it, to convert decimal degrees to degrees, minutes, and seconds.

To gather the latitude and longitude in decimal degrees, go to the Earth Explorer site and click on the US Features option in the left sidebar. For cities outside the US, you can click on World Features next to it. Beneath these options, type in Los Angeles in the search box and select California from the drop down menu for the state. The latitude and longitude coordinates will come up below. They are:

• Latitude: 34.0522° (positive latitude numbers indicate N)

• Longitude: -118.2437° (negative longitude numbers indicate W)

Using a calculator, we find that the degrees, minutes, and seconds are:

• Latitude: 34° 03′ 8″ N

• Longitude: 118° 14′ 38″ W

Now we have what we need to start making some adjustments to the time. To do so, we’ll be using the time zone offset information we found. In the next lesson, we’ll look at what we can do if, for any reason, we don’t have the time zone information but do have the longitude. That will add to what I talk about in the video accompanying this lesson.

Time Zone Adjustments

Daylight Savings Time Adjustment

If an event occurs during daylight savings time, the first step is to convert the event time to standard time. I offer an alternative in the section below called, A Consideration, but let’s do it this way, first, as this is how I learned it from Dave Campbell’s videos⁶. We have determined that daylight savings time will be in effect in Los Angeles for the Virgo New Moon, so we’ll make that adjustment.

The date and time for the Virgo New Moon for Los Angeles, California as it was given to us is:

• September 14, 2023, 18:40

Because we had to add an hour to get to daylight savings time, we need to subtract it to adjust the time back to standard time.

• 18:40 - 1 hr = 17:40

With this adjustment back to standard time, there is no change to the date, so for this step, we get the standard date and time of:

• September 14, 2023, 17:40

Greenwich Mean Time Adjustment

Now that we have the event date and time for the standard time zone of the location, we can adjust the event time (and date, if necessary) to Greenwich Mean Time. In other words, we want to figure out what time it was in Greenwich at the standard time of the event. Once we’ve worked this out, we can find the sidereal time from an ephemeris and start our sidereal time calculations. We’ll do that in the next lesson or the one after. For now, we’ll work out the Greenwich Mean Time.

Since we know from our found data that the standard time zone for Los Angeles (PST) is 8 hours behind (-8) GMT, to convert it so that it is the same time as GMT, we add 8 hours to the standard time for our event. Thus, we get:

• Pacific Standard Time of Event: 17:40

• Plus GMT Offset: 8 hrs

• Result: 25:40

Because our answer is greater than 24, that means that when we added 8 hours to the standard event time, we pushed the date over to the next day. So we take 24 hours from our result and add that as a day to the date.

• 25:40 - 24:00 = 01:40.

• September 14, 2023 + 1 day = September 15, 2023.

So the GMT date and time for the Virgo New Moon is:

• September 15, 2023, at 01:40

We are now set up to make the leap to sidereal time and refine our result a little more, but we’ll do that in the next lesson or the one after. It depends on how much space I have to give to the math for decimal degrees and degrees, minutes, and seconds.

A Consideration

When We Have the Daylight Savings Time Offset

Because Time.is gives us the daylight savings time offset of -7 hours, we could simply add 7 hours to the time and date we were given of September 14, 2023, 18:40. When we do that, we get:

• 18:40 + 7 hrs = 25:40

From here, we do the same thing we did above. Because our result is over 24 hours, we need to subtract 24 hours from the time and add that as a day to the date, as follows:

• 25:40 - 24 hrs = 01:40

• September 14, 2023 + 1 day = September 15, 2023

Thus, we get the same result as above:

• September 15, 2023, 01:40

As noted previously, the method taught by Dave Campbell in his video series is to make the local daylight savings time adjustment in a distinct step.⁷ I assume there are reasons for that, whether historical or because there are certain circumstances that might come up that would warrant doing that step first. Another reason might be that it goes back to using longitude to work out the needed time. That’s a guess, but we’ll look at longitude calculations in the next lesson.

Worksheets

I use Notion as one of my online workspaces. To help me keep track of the data and calculations for the Virgo New Moon chart example, I am creating some simple tables to record the data as I go so I have it all in one place. I thought it might help if I made the page public so people could follow along as information and calculations for each lesson are added. You can find the page here. I will be adding more tables as we cover the material. I also include a link to a page with blank tables that people can duplicate and use if Notion is a platform that works for them and the tables seem useful. As with my working version, I will be adding blank tables to the duplicate-able version as we cover the material.

Astrology Journal Bookshop

I now have an online bookshop, Astrology Journal Bookshop. It is an affiliate of Bookshop.org. Bookshop.org is a certified B company that was founded to support independent booksellers. When you purchase a book from my shop by using the link I provided above (or landing on my bookshop by some means and purchasing a book), I earn a commission. The selling point for me is that some of the proceeds from your purchase also go to support independent booksellers. If you would like a way to support me that also supports independent booksellers, please consider visiting my shop to purchase your astrology books. I will continue to add more books to the store over time.

For more information, see my bookshop announcement at the end of the previous lesson. There, I link to some reviews of Bookshop.org that might be helpful. You can also find out more about Bookshop.org on their About page.

Edit 11/25/2023: I forgot to include the image credit. It is public domain, but it’s nice to know where it came from: By UnaitxuGV, Heitordp and others - Outline based on the map by the U.S. Central Intelligence Agency (CIA).Time zone information based on the Time Zone Database by the Internet Assigned Numbers Authority (IANA), with the following differences:Mongolian provinces of Govi-Altai and Zavkhan in UTC+8[1][2][3][4][5]Abkhazia, South Ossetia and Russian-occupied territories of Ukraine in UTC+3[6][7][8], Public Domain, https://commons.wikimedia.org/w/index.php?curid=22556731

Corrections

8/5/2023

• I forgot to include the link to Griffith Observatory’s page with the Moon phase tables for 2023, so I added that in.

• The decimal degree conversion calculator that I linked to is on the Federal Communication Commission’s website, not the Federal Trade Commission’s. I corrected that.

• I changed the name of this section from Updates to Corrections to better reflect what it is.

Previous Articles in the Series

Part 1:

Course Introduction: Astrology Chart Calculation

Part 2:

Why Do Charts Need to Be Calculated?

Part 3:

Resources \ Introduction to Calculating Charts by Hand

Resources

Campbell, D., AFA [Dave Campbell]. (2013, January 13). How to calculate your birth chart by Dave Campbell AFA part 1 of 5 [Video]. Youtube. (This is the first in a series of 5 videos on chart calculation. I link to all of them in my article, Resources \ Introduction to Chart Calculation by Hand).

EarthExplorer. (n.d.). USGS: Science for Changing the World. https://earthexplorer.usgs.gov/

Griffith Observatory. (2021, December 30). 2023 Phases of the Moon - Griffith Observatory - Southern California’s gateway to the cosmos! https://griffithobservatory.org/explore/observing-the-sky/whats-in-the-sky/the-moon/2023-phases-of-the-moon/

Mangan, E. (2023b, July 27). Resources \ Introduction to Chart Calculation by Hand. Astrology Journal. https://astrologyjournal.substack.com/p/resources-introduction-to-calculating

Mangan, E. (2023, July 18). Why Do Charts Need to Be Calculated?: Part 2: Astrology Chart Calculation Course. Astrology Journal. https://astrologyjournal.substack.com/p/why-do-charts-need-to-be-calculated

Shanks, T.G. (1990). The American Atlas (5th ed.). ACS Publications.

Time.is. (n.d.). https://time.is/

Ward, K. (n.d.). Astrology: Calculating the Chart - UT or GMT. Trans4mind^TM. https://trans4mind.com/personal_development/astrology/Calculations/calcGMT.htm

Wikipedia contributors. (2023). List of time zone abbreviations. Wikipedia. https://en.wikipedia.org/wiki/List_of_time_zone_abbreviations

Mentioned in the Notes

These are sources I mentioned in the footnotes but not in the body of the article.

Watched or Read

Brennan, C., & Ribeiro, L. (2021, July 31). House Division Calculations in Astrology Explained (episode 313). The Astrology Podcast. https://theastrologypodcast.com/transcripts/ep-313-transcript-house-division-calculations-in-astrology-explained/

Wikipedia contributors. (2023). House (astrology). Wikipedia. https://en.wikipedia.org/wiki/House_(astrology)

Not Read

Brennan, C. (2017). Hellenistic Astrology: The Study of Fate and Fortune. Amor Fati Publications.

Holden, J. (1982). Ancient House Divisions. American Federation of Astrologers, 1(1), 19–29. https://www.hellenisticastrology.com/holden-ancient-house-division.pdf. (The link is to the pdf version of the article housed on the hellenisticastrology.com website, which is the version I found. Retrieved on July 29, 2023.)

Houlding, D. (2006). The Houses: Temples of the Sky. The Wessex Astrologer.

Murphy, P., & Rosato, B. (1998). The Math of Astrology: A Step by Step Guide. American Federation of Astrologers.

Notes

1. Here are some resources that might be helpful if you’re interested in diving into the technical aspects of house divisions.

• For a general overview of the topic of houses, including house systems, see the Wikipedia entry, House (astrology)

• For an in-depth conversation on the math \ astronomy of the different house systems, see The Astrology Podcast episode, House Division Calculations in Astrology Explained. I would recommend scrolling down and watching the video version, at least for the part where they go over the technical aspects, as they use helpful visuals.

• Some books and articles cover the topic of houses from a historical and/or technical perspective, none of which I have read at this time. Below are the ones I’m currently interested in.

  • Deborah Houlding’s book, The Houses: Temples of the Sky

  • Chris Brennan’s book, Hellenistic Astrology (he has a couple of chapters devoted to the topic)

  • James Holden’s article, Ancient House Division (available for free in pdf form on one of Brennan’s websites, Hellenisticastrology.com)

2. For the steps that involve calculations, I am mostly following the approach presented by Dave Campbell (AFA) in his Youtube video series on how to calculate charts by hand, but I am also incorporating things I learned from Ken Ward’s article tutorials on how to calculate charts. And I am adding my own perspective and filters, such as the umbrella for the types of data, which helped me to organize what I was learning. Note that the link above to Campbell’s video series goes to the first video. You can find the links to all of the videos in the previous article covering resources.

3. In his videos, Campbell is following the process presented in the book, The Math of Astrology by Peter Murphy and Beth Rosato. I have not read it but it should be noted as a source and resource.

4. I chose the Virgo New Moon in September because, following the lead of Campbell in his video series on chart calculation, I wanted an event date and time that will require some date adjustments that will be helpful to see. The example he uses requires a date adjustment. It also has an interesting time zone challenge, so I recommend doing his tutorial, too. But I’d start with one and work through it before doing the other. See note 2 for links to his videos.

5. The US Naval Academy’s longitude and latitude finder, Find Latitude and Longitude, abides by Google’s terms and conditions. Because I am on a search for sources with information that I can use freely, I realized I wanted to find an alternative. But I still really like the their web app.

6. For links to Campbell’s first video and to the previous article in this chart calculation course, which has links to all the videos, see footnote 2.

7. Campbell goes over the first part of the process of making the daylight savings time adjustment in his first video (see note 2 for link). For links to all the videos in his series on chart calculation, see the previous article in this course on resources, linked to in footnote 2.