Question: If the birth time is off by 2 minutes would that change the reading much?
2 minutes changes the ascendant slightly. Each minute of time changes the ascendant by about 13 minutes of arc (there are 60 minutes in a degree, 30 degrees in a sign). So it is very slight.
What might change is the ascendent signs of a few of the amshas… this affects some interpretations quite subtly. The biggest change, and the most important to note would be in the smallest subdivision, the 60th subdivision (ṣaṣtyaṁśa).
A few more minutes and we start to more significantly change how the planets align in relation to the asc, which changes the relative prominence of the planets. This plus accumulating even more changes in the subdivisions causes pretty significant difference in interpretations when you get to around 5-10 minutes change in birth time (On average. Every chart has different sensitivity, because in some charts the starting assumption of birth time is close to major borders, like in the zodiac or a large division of the zodiac).
And, of course, here I am assuming no change in location. If there is also change in location, that is another agent changing the chart.
Question: Are western harmonic charts the same as the aṁśa subdivision charts of Vedic astrology?
From what I know about harmonic charts, no – they are quite different from the Vedic aṁśā charts.
As far as I know, in a western harmonic chart you simply multiply a planet’s degree by a specific harmonic number (like 9, for example), and then subtract 360 from that product until you have a number below 360 and this is considered the planet’s placement in that harmonic.
In the aṁśa charts of Vedic astrology, you don’t move the planets, you just divide the signs into subdivisions. For example for the 9th divisional chart (navāṁśa) you divide each sign into 9 equal parts. Each subdivision is assigned a zodiac sign. Usually the rule for this asignment is that the first division of Aries starts with Aries, and then just assign signs to the divisions in order, repeating 9 times to fill up the whole 360 degree zodiac. Then, you draw a new chart by looking at the sign of the subdivision that the ascendant lies in, and making that the ascendant of the divisional chart. Then looking at the sign each planet occupies in their subdivision, and marking the planet in that sign in the subdivision.
This is my ascendant, at about 25 degrees of Capricorn. If you look at the 9th divisional subdivisions on the outer border of the ring, you’ll notice that this puts my ascendant in a Leo subdivision. So when you draw my 9th divisional chart, you would put the ascendant in Leo, making Leto the first house. And you do the same for every planet, creating a complete subchart.
For example, here are my Sun and Mars, they are both close together early in Leo. And as you can see, this puts them together in a Taurus subdivision. So in the 9th subdivision chart, you would mark the Sun and Mars as being in Taurus.
So my Navamsha winds up looking like this:
Here you can see Leo as the first house, and the Sun and Mars together in Taurus, which is the 10th house because it is the 10th sign from Leo.