House cusp discrepancies

Hi, folks --

I'm just now learning to calculate house cusps by hand. It's sort of fun, in a twisted kind of way.

Anyway, I've run into a little problem. I wanted to practise with some charts before doing my correspondence course's homework, so I decided to calculate the house cusps for an Astrodienst chart which I already have. The Astrodienst chart says "Method: Web Style/Placidus," and Placidus is the house system my course uses.

The birth data is September 8, 1952, 5:12 a.m., Paterson, New Jersey, U.S.A., 40 N 55, 74 W 10, daylight savings time.

Now, Astrodienst and I both agree that the sidereal time is 3 hours, 24 minutes, and 50 seconds.

When I look up this sidereal time in my Placidus Book of Tables, the nearest box is for 3 hours, 24 minutes, 0 seconds (the next one up is 3 hours, 28 minutes, 0 seconds). The MC for that box is 23 Taurus 23. But on the Astrodienst chart, it's listed as 23 Taurus 36. The other house cusps are similarly off, sometimes by degrees, for instance my calculations say the 3rd house should be 25 Libra 31, and Astrodienst says 20 Libra 27.

I realize the calculation I'm doing is not as precise as Astrodienst's computer calculation, but these discrepancies seem to be way more than they should be. What totally confuses me is that Astrodienst and I agree on both the sidereal time and the latitude, as well as the house system.

Please, somebody tell me what silly mistake I'm making!

Thanks --
Lee

Well you don't seem to have allowed for the difference between the 'box' value and the actual ST of birth. The birth took place 50 seconds later. So you need to interpolate between the two box values for 50 seconds and add that to the lower box value.

I haven't the same tables as you but this is how I'd proceed.

For 23 Taurus I have an ST of 3 hours 22 mins 24 secs

For 24 Taurus I have an ST of 3 hours 26 mins and 30 secs

The difference between these two is 4 mins and 6 secs - 246 secs

The difference between the earlier ST and the desired ST is 2 mins and 26 secs (146 secs). So I would add 146/246 of a degree to 23 to get the MC. This is just over 0.59 of a degree or 35.6 minutes. This gives me an MC of 23 degrees 35 mins (or if you want to round up 23 degrees 36 mins).

You can then use this MC to calculate an Ascendant but remember that you may also need to interpolate for latitude if you are using tables. The Cusps should then adjust to the correct MC and Ascendant

Using tables will never be as accurate as a computer, as you say - unless the program has bugs (and I've met ones that have). Enjoy your calculations - knowing how the chart is put together is extremely useful, even if you end up using a computer in the future.

Thanks, Minderwiz, I see my mistake now. Joanne Wickenburg says on the tape that more accurate calculations will be taught later, and I assumed she meant the calculations to figure house cusps to the second, but I think what she meant was the procedure you outlined. So, I was jumping the gun by thinking that my calculations at this point would result in an accurate chart.

I do find that understanding how to do the calculations by hand really does help in understanding what's going on in a chart. I recommend it!

-- Lee

Lee,

I think your approach of comparing your calculations to a computer generated chart is excellent. I did the same when I was learning and it enables you to track down errors if there is a difference between the two - as in this case. Indeed this was how I discovered that one progam actually was inaccurate - double checking against my calculations and a second program which agreed with me.

One thing to watch out for is that sometimes there can also be small discrepencies between the Longitude and Latitude given for Cities or other places by various Atlases, either computer based or book based. This can lead to differences of one or two minutes of arc in MC and ASC. There's nothing worse than doing the calculation four times and then finding that the reason the figures don't match the computer is that it is using slightly different co-ordinates than those given in the question you are answering! So it is often better to enter the co-ordinates manually from your question than rely on the computer's Atlas.