Most individuals know their Sun signs, some even know their Moon signs, but if you've never had your chart calculated, you couldn't know your ascending sign. Unless you're a math whiz, learning to hand calculate a chart is difficult and learning how to calculate the ascending sign and degree is just the start. This is the mathematics of geocentric astrology that every astrologer has to learn in order to pass the certification examinations given by the major astrological organizations.
Step 1: Prepare for Your Calculations
In addition to paper, pencil, eraser, and calculator, you'll need three books to calculate the ascending sign and its exact degree. If the books listed are available for reference online, links are provided. Take your time with each step and be clear about what you've done before moving on to the next.
Get an Atlas
A World Atlas is a collection of various maps of the Earth. It shows the longitude and latitudes of any location. You'll need an atlas to pinpoint the latitude and longitude of the birth or event location. They are quite expensive to buy, but you can find one at your local library. Astrotheme provides a free atlas for astrology that gives latitude, longitude, time zone, and universal time (GMT) for any given city and date. These are all things you will need to know to calculate an ascendant.
Find a Table of Houses
Table of Houses is a book astrologers use that contains tables showing the degrees of the signs which occupy the cusps of the houses at the different latitudes. You will need this book to interpolate the exact midheaven and ascendant. The best table of houses available is The Michelsen Book of Tables: Koch and Placidus Table of Houses. However, a Simplified Scientific Table of Houses can be found in pdf form online for free.
Obtain the Midnight Ephemeris for Year of Birth
Midnight Ephemeris an almanac that contains information about sidereal time and the positions of Sun, Moon, planets, and various other points at 12:00 am on the day in question. You can find Ephemerides for years 1900 to 2100 online at Find Your Fate.
Keep in Mind as You Work
It will help for you to know the following:
- Complete and accurate astrological charts are based on the exact location and exact time of a birth or event.
- Interpolation is defined as finding a specific point between two points, and that is what these calculations are about.
- Latitude is measured in degrees north or south of the equator. The equator is zero degrees latitude. Latitudes circle the globe parallel to the equator.
- Longitude runs from pole to pole. The lines of longitude circle the globe in a north-south direction. Zero degrees longitude runs through the Royal Observatory in Greenwich, England.
- Standard meridians are lines of longitude that are 15 degrees apart. Each of the 24 time zones has what's called a standard meridian that is used to set the uniform time throughout each zone. They begin at zero degrees longitude or the Royal Observatory in Greenwich, England and circle the globe every 15 degrees of longitude.
- Degrees of latitude and longitude can be divided into 60 parts called minutes. Each minute can be divided into 60 parts called seconds.
You can covert degrees to time:
- 1 minute is equal to 4 seconds of time
- 1 degree is equal to 4 minutes of time.
- 7.5 degrees is equal to 30 minutes of time.
- 15 degrees is equal to 60 minutes of time.
- 360 degrees is equal to 24 hours of time.
The Different Methods of Timekeeping
The following methods of timekeeping are used throughout the first steps and are important for you to understand.
- Local mean time (LMT) is a method of timekeeping that uses the Sun's movements across the sky. It's based on the average length of a solar day. Timeanddate.com clearly defines LMT.
- Greenwich mean time (GMT), also called universal time (UT), is international time and the basis of the world time clock. GreenwichMeanTime.com gives a complete explanation and also provides a converter.
- Military time is not based on am or pm. It's based on a 24-hour clock and is a method of timekeeping in which each day is from midnight to midnight. It's divided into 24-hour increments instead of 12-hour increments. This military time conversion chart makes it simple to take conventional am/pm time and quickly convert it to military time.
- Sidereal time (Sid T) is a timekeeping system that astronomers use to locate celestial objects. Briefly, it's a time based on Earth's rate of rotation measured relative to the fixed stars rather than the Sun. Physics and astronomy has a sidereal clock that compares local time to Sid T. The Sid T relevant to the day of birth can be found in the Midnight Ephemeris.
A Bit of Advice
Before you move on to the steps below, take the time get a feel for the books by checking the links provided. Likewise, get a clear understanding of everything above. Doing this will make the calculations you're about to begin much easier to understand and complete.
Step 2: Gather Birth Data
Every astrologer's first question to a client is about accurate and complete birth data. The reason for this is that the more exact the birth data, the more accurate a calculated birth chart will be, and the more accurate the astrologer can be in their interpretation and forecasting.
On a piece of paper, list the month, day, year, time of birth, place of birth, and the time zone in use at the time of birth.
1. Find the longitude and latitude of the birth location in the atlas.
2. Find the Standard Meridian (SM) of the time zone in use. (Reminder: The SM is used for reckoning standard time throughout most of the world. They are those longitudes that are exactly divisible by 15 degrees (30, 45, 60, 75, 90, etc degrees).
- Jane Doe was born on October 14, 1950, 12:05 pm Eastern Standard Time (EST), Atlanta, Georgia.
- The longitude of Atlanta Georgia is 84W23. The latitude is 33N45.
- The SM for EST is 75 degrees longitude.
Step 3: Calculate Local Mean Time of Birth
Few individuals are born exactly on the degree of a time zone's standard meridian longitude, so to find the local mean time (LMT), you have to adjust for the exact longitude of the place of birth.
Keep in mind that one hour equals 15 degrees of longitude. Each degree of longitude can be divided into 60 minutes, and each minute into 60 seconds. One degree of longitude equals 4 minutes of time. One minute of longitude equals 4 seconds of time. (See above).
1. Write down the standard meridian longitude degree for the time zone of the birthplace.
2. Determine the middle of the time zone by adding 7.5 degrees to the standard meridian. (Reminder: 15 degrees equals one hour so 7.5 degrees equals ½ hour).
3. Subtract that amount if the place of birth is west of the middle of the time zone and add if east.
4. Multiply 4 minutes times the degrees of longitude and 4 seconds times the minutes of longitude.
- Eastern Standard Time (EST) begins at 75 degrees longitude.
- Add 7.5 degrees to 75 degrees. This gives you 82.5 degrees of longitude. Atlanta, Georgia's longitude is 84 degrees; this is west of 82.5 degrees so you must subtract.
- The difference between 75 degrees and 84W23 is 9 degrees 23 minutes.
- Four minutes multiplied by 9 degrees converted to time equals 36 minutes.
- Four seconds multiplied by 23 minutes converted to time equals 92 seconds.
- The longitudinal difference between the time at the SM for EST and the longitude of birth is 36 minutes 92 seconds, which converts to 37 minutes and 32 seconds.
- Birth time of 12:05:00 pm EST minus 37 minutes and 32 seconds equals a LMT of 11:27:28 am.
5. Convert LMT to military time (MT), do nothing if time is AM. Add 12 hours if time is PM.
6. If relevant, adjust for Daylight Savings Time, etc.
- This is a morning birth so there is nothing to do in regards to MT.
- No other time adjustment was in effect.
LMT of birth is 11:27:28 am.
Step 4: Find Greenwich Mean Time of Birth
The 24 time zones are based on Greenwich mean time (GMT). Those 12 time zones east of Greenwich are minus zones so the number of hours would be subtracted from the LMT of birth. Those 12 time zones west of Greenwich are plus zones and would be added to LM birth time.
Reminder: Standard meridians are those lines of longitude, 15 degrees apart, that are used to set the uniform time throughout each time zone.
1. Divide the SM longitude (see step 3) that the time zone is based on by 15. The answer is the number of hours different your time zone is from GMT.
2. Add your answer to or subtract it from LMT, depending if your time zone is east or west of Greenwich, England.
- Eastern Standard Time (EST) for Atlanta, Georgia is based on the 75 degrees longitude SM. That's 5 hours (75 divided by 15 equals 5).
- Atlanta, Georgia's longitude is west (84W33) of Greenwich, England, so it's GMT+5 and you would add the 5 hours to the LMT.
- LMT of birth 11:27:28 plus 5 hours equals 16:27:28 GMT.
GMT of birth is 16:27:28.
Step 5: Find Calculated Sidereal Time of Birth
It's now time to get out and open up your Midnight Ephemeris so you can find out the sidereal time at midnight on the day of birth.
Reminder: An Ephemeris an almanac that contains information about sidereal time and the positions of Sun, Moon, planets, and various other points.
1. Find the page in your ephemeris that correlates to the year and month of birth. Go down the page to the day in question and in the first column (Sid Time) you'll find the sidereal time at midnight on that day. Write that Sid Time down.
2. Divide the degrees of birth longitude by 3 and multiply by 2. Write the answer down.
3. List LMT.
4. Make what's called an accelerated interval correction by multiplying the number of hours in LMT by 10 seconds and dividing the number of minutes in LMT by 6 seconds. Add those two answers together. Write that answer down.
5. Now, you're going to add the answers to all four steps above together. (Remember to keep columns straight in terms of hours, minutes, and seconds.)
6. If the total hours in your answer is 24 hours or greater, subtract 24 from the hours.
- 01:27:53 (Sid time from ephemeris)
- 00:00:56 (84 degrees of longitude divided by 3 multiplied by 2)
- 11:27:28 (LMT)
- 00:01:55 (11 time 10 equals 110) (27 divided by 6 equals 4.5) (110 plus 4.5 equals 114.5 or 115 minutes) (115 divided by 60 equals one minute 55 seconds or 00:01:55)
- 12:55:19 (converts to 12:58:11)
- Since the total number of hours is not greater than 24 nothing more needs to be done.
Calculated sidereal time of birth is 12:58:11
7. List all these times together for future use.
Birth time of 12:05 pm EST, Atlanta, Georgia correlates to:
- LMT: 11:27:28
- GMT: 16:27:28
- Calculated Sid. Time: 12:58:11
Step 6: Interpolating the Exact Midheaven
Interpolation is finding a specific point between two points. As an example, you could be trying to find a specific degree and minute that lies between 15 degrees of Libra MC and 16 degrees of Libra MC. You'll need the Table of Houses to interpolate that specific point. The midheaven (MC) is the zenith, the highest point of the 360-degree astronomical circle called the zodiac.
Reminder: Table of Houses is a book astrologers use that contains tables showing the degrees of the signs which occupy the cusps of the houses at the different latitudes and Sid times.
1. From Table of Houses, look up the Sid time previous to the calculated Sid time of birth.
2. Look down the latitude columns and find the latitude of the birth location and write down to corresponding MC.
(Note: In the Simplified Scientific Tables of Houses mentioned above, you'll have to locate the latitude of birth first and look down for the Sid times previous to and following the calculated Sid time of birth.)
Atlanta's latitude is 33N45.
- Calculated Sid time of birth 12:58:11.
- From Table of Houses, Sid time previous to the calculated Sid time is 12:55:15.
- Look down the latitude column for 33N. This shows a 15 Libra MC.
3. Write down the calculated Sid time of birth and subtract the Sid time previous to birth that is listed in the Table of Houses from the calculated Sid time.
4. The answer is the elapsed time from the Sid time previous to birth to calculated Sid time of birth.
5. Convert to seconds.
- The calculated Sid time of birth is 12:58:11.
- The Sid time previous to birth from Table of Houses is 12:55:15 (15 Libra MC).
- Subtract 12:55:15 from 12:58:11. The answer is 00:02:56.
- Converted to seconds, it is a 176 second elapsed interval.
6. Find the Sid time following the calculated Sid time of birth from Table of Houses and write down the corresponding MC and Sid time.
7. Subtract from Sid time following birth from the Sid time previous to birth.
8. Convert to seconds.
- Sid time following the calculated Sid time of birth from table of houses for 16 Libra MC is 12:58:57.
- Sid time previous to the calculated Sid time of birth for 15 Libra MC, is 12:55:15.
- 12:58:57 minus 12:55:15 equals 00:03:42.
- 00:03:42 converted to seconds is a 222 second elapsed interval.
9. Divide the smaller interval by the larger to find the ratio.
10. Convert the ratio to minutes.
11. Add to previous MC.
- 176 divided by 222 equals .79 ratio (remember this ratio because you're going need it again).
- 60 multiplied by .79 equals 47.56 minutes or 48 minutes (Remember: One degree equals 60 minutes).
- 15 Libra plus 48 minutes equals 15 Libra 48.
Exact MC is 15 Libra 48.
Step 7: Interpolating the Exact Ascendant
The ASC is the point of intersection of the ecliptic (the Sun's path) with the eastern horizon of the birthplace. There are exactly 30 degrees per astrological sign; added together they equal 360 degrees, the span of the ecliptic. All twelve signs rise over the ASC during the course of one 24-hour day. Ascending signs vary by location and change approximately every two hours. The degree of the ascending sign changes approximately every four minutes.
Once again, you're going to be finding a specific point between two points. As an example, you could be trying to find a specific degree and minute that lies between 27 Sagittarius 14 and 28 Sagittarius 04. You'll need the Table of Houses opened to the same page as in step 6 to interpolate the exact degree and minute of the astrological sign that was rising in the east at the time and place of birth.
1. Find the percentage of correction for minutes of latitude. Divide the minutes of birthplace latitude by 60 to find the percent of correction. (Reminder: One degree equals 60 minutes).
Latitude for Atlanta, Georgia is 33N45, 45 minutes divided by 60 equals .75. (This is the percent of correction of latitude from 33N to 34N.)
2. Consider calculated Sid time of birth and locate the Sid time in the Table of Houses that it falls between. Look down the latitude column until you find the birthplace latitude. Write down the corresponding sign and degree of the ASC shown for both the Sid time previous to birth and the Sid time following birth. Find the difference between the two and write that down.
- Calculated Sid time of birth was 12:58:11. The latitude of Atlanta, Georgia is 33N.
- The previous Sid time at 33N latitude shows 15 Libra MC and a 27 Sagittarius 40 ASC. The following Sid time at 33N shows a 15 Libra MC and a 27 Sagittarius 05 ASC. The difference is 35 minutes.
- The following Sid time at 34N latitude shows 16 Libra MC and a 28 Sagittarius 31 ASC. The following Sid time at 34N shows a 16 Libra MC and a 27 Sagittarius 55 ASC. The difference is 36 minutes.
3. Multiply the difference of both by the above percent of correction, then subtract.
- The percent of correction for 33N45 latitude is .75.
- 15 Libra: 35 minutes multiplied by .75 equals 26.25 or 26 minutes. (previous)
- 16 Libra: 36 minutes multiplied by .75 equals 27 minutes. (following)
- Subtracting 26 minutes from 27 Sagittarius 40 results in a previous to birth Asc of 27 Sagittarius 14.
- Subtracting 27 minutes from 28 Sagittarius 31 results in the following birth Asc of 28 Sagittarius 04.
Note: The exact ASC always falls somewhere between the previous Sid time's corresponding ASC sign and degree and the following Sid times's corresponding ASC sign and degree. In this example, the exact ASC will fall somewhere between 27 Sagittarius 14 and 28 Sagittarius 04.
4. Subtract the previous ASC sign and degree from the following ASC sign and degree.
5. Multiply the answer by the ratio found above in Step 6.
6. Add that answer to previous ASC sign's degrees and minutes to find the exact ASC.
- Subtract 27 Sagittarius 14 (previous) from 28 Sagittarius 04 (following). The answer is 50 minutes.
- 50 minutes multiplied the .79 ratio equals 40 minutes.
- Add 40 minutes to the ASC sign and degree previous to birth for 33N latitude, 27 Sagittarius 40.
- 27 Sagittarius 40 plus 40 minutes equals 27 Sagittarius 80'. This converts to 28 Sagittarius 20.
Exact ASC is 28 Sagittarius 20.
The Cross of Matter
The Cross of Matter is the backbone of any astrological chart. This is where things happen. It's composed of four angles. The ASC is exactly opposite the descendant (DC) and the MC is exactly opposite the Imum Coeli (IC). When you've found the exact sign and degree of the ASC, you'll also have found the signs and exact degrees of the Cross of Matter.
The four angles for Jane Doe born on October 14, 1950, 12:05 pm EST in Atlanta, Georgia are:
- ASC 28 Sagittarius 20
- DC 28 Gemini 20
- MC 15 Libra 48
- IC 15 Aries 48
Now, if you want a fully calculated astrological chart, all that's left to do is to determine which house system you're going to use, open your Midnight Ephemeris to year and month of birth, look down the column to the day of birth, list the signs and degrees of the Sun, Moon, planets and other important points, and begin interpolating the exact position of each at the time of birth.
Try, Try Again
Unless you're mathematically inclined, calculating an ascendant isn't easy, even with step-by-step directions. So, don't get discouraged if you don't get it right the first few times. Try, try again. Of course, if math just isn't your cup of tea and these calculations just seem too complicated and overwhelming, you can find the ascending sign and your entire calculated chart for free at many spots on the internet. That will certainly be a lot less stressful and quicker, but you'll also miss out on a wonderful opportunity to gain greater insight into the importance of astrological geo-positioning.