A Great Coordinate Conversion Tool for Astrologers

The accuracy of astrological charts depends on having a fairly precise time of birth at a specific location on earth. Most astrological software allows you to select a city of birth but does not provide the exact coordinates for a specific address within that city.

Fortunately there are websites that allow you to identify a specific address on a map to determine its exact coordinates (geographic latitude and longitude). One such site, which I recently came across, is called the NGS Coordinate Conversion and Transformation Tool and is located at https://www.ngs.noaa.gov/NCAT/.

Here is an example. I was born at Waterbury Hospital and wanted to know the exact coordinates of the hospital. Using the site, I generated the following map and set of coordinates:

As you can see, you can move the marker on the map to a specific location and then press “Submit” to calculate its exact coordinates. It turns out that Waterbury Hospital has coordinates: 41N33’33.44″ latitude and 73W03’32.94″ longitude, which you can then enter into your astrological software to generate a precise Ascendant and house cusps for your time and place of birth.

Note that this site returns the “geodetic” or geographical latitude of the location based on the ellipsoid shape of the Earth which, like many of us humans, bulges a bit at the Equator and is not a perfect sphere as can be seen in this diagram from wikipedia.

Some astrologers prefer to convert the geodetic (geographical) latitude into its geocentric equivalent, which can be up to about 12 arc-minutes less than the geodetic, depending on the geodetic latitude. The program Solar Fire, for example, offers an option to use geocentric latitude and gives the following explantion:

“Apply Geocentric Correction to Latitude

It is possible to calculate charts based either on geographic (also known as geodetic) latitude or geocentric latitude. Atlases such as the ACS International Atlas and the ACS American Atlas contain geographic latitudes. Geographic latitudes are based on the measurement of the angle of the local horizon (also known as the geodetic horizon) onto the celestial sphere. It is assumed that all latitudes entered into Solar Fire are geographic latitudes, and all displayed latitudes, such as in chart details text, are geographic latitudes.

However, because the earth is not perfectly spherical it is also possible to define a latitude based on where the horizon would be if the earth was a perfect sphere, which also corresponds to the angle of a line from that location to the center of the earth. This is known as a geocentric latitude. The geocentric latitude is never more than 12 minutes of a degree different from the geographic latitude. There are arguments for and against using each of these types of latitude. Astrologers are divided about which is the most appropriate to use.

Solar Fire always displays latitudes as a geographic latitudes. However, when this option is on, any chart calculations performed will have a correction applied to the latitude to convert it from geographic to geocentric. Any chart that has had this correction applied will have a “G” appended to its latitude. For example, a geographic latitude of 34°S55’ is displayed as 34°S55’G when this option is on. In order to determine exactly what the geocentric correction is, you must view a “Chart Analysis” report for the chart. If the geocentric correction has been applied, then this will be specified in the report, and the corrected geocentric latitude will be shown as well as the originally entered geographic latitude.”

The next graph illustrates the amount of correct in arc-minutes which must be subtracted from the geodetic (geographic) latitude to arrive at the geocentric latitude based on the 1984 World Geodetic System revision (WGS 84):

The straight line in blue signifies degress of geodetic latitude from 0 to 45.
The curved red line, which ranged from 0 to about 12 on the y-axis, shows the number of arc-minutes to be subtracted from the geodetic degrees to arrives at the geo-centric latitude.

For example, suppose your geodetic latitude is about 30 degrees North, then 30 degrees along the x-axis corresponds to a correction factor of about 10 arc-minutes on the y-axis. Hence, your geocentric latitude (based on the center of a perfectly spherical Earth) would be 30 degrees N minus about 10 arc-minutes. If we were to calculate it using the formulas that underlie WGS 84, we would get a result of 29d 50m 01.1s North geocentric latitude, which is about 10 arc-minutes less than the geodetic latitude of 30 degrees North.

This is an alternate form of the above graph. The horizontal blue lines represent geodetic latitudes, The shorter red lines represent the number of arc-minutes (x-axis) which need to be subtracted from the geodetic latitude to calculate the geocentric latitude.

The following table presents the same information as the above graph. Subtract the arc-minute correction from the geodetic degree to arrive at the corresponding geocentric latitude in the WGS 84 system.

Degree (Geodetic)	Arc-minute Correction	Degree (Geodetic)
0	0	90
1	0.4	89
2	0.8	88
3	1.2	87
4	1.6	86
5	2	85
6	2.4	84
7	2.8	83
8	3.2	82
9	3.6	81
10	3.9	80
11	4.3	79
12	4.7	78
13	5	77
14	5.4	76
15	5.8	75
16	6.1	74
17	6.4	73
18	6.8	72
19	7.1	71
20	7.4	70
21	7.7	69
22	8	68
23	8.3	67
24	8.6	66
25	8.8	65
26	9.1	64
27	9.3	63
28	9.6	62
29	9.8	61
30	10	60
31	10.2	59
32	10.4	58
33	10.6	57
34	10.7	56
35	10.8	55
36	11	54
37	11.1	53
38	11.2	52
39	11.3	51
40	11.4	50
41	11.4	49
42	11.5	48
43	11.5	47
44	11.6	46
45	11.6	45

Find the geodetic degree of latitude in the left or right column and subtract the number of arc minutes in the center column to calculate to corresponding geo-centric latitude.

In the example of Waterbury Hospital at the beginning of this post, we can see that it has a geodetic latitude of 41d 33m 33.44s, which would correspond to a geo-centric latitude of 41d 22m 06s if the Earth were a perfect sphere and the latitude of the hospital were measured from the exact center of this sphere.

Addendum (26 Sep 2020): A similar and very useful site for finding the geographic latitude and longitude of a location is https://www.findlatitudeandlongitude.com/

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All original material in this post is copyright Anthony Louis 2020.