How to Plot & Read a Natal Chart

Local clock time is first converted to Universal Time (UT) by removing the time-zone offset and any daylight-saving adjustment. This gives a single, unambiguous instant that astronomers worldwide agree on — the anchor for everything that follows.

Internally this UT instant is expressed as a Julian Date (a continuous day-count) so the position of every body can be looked up from an ephemeris.

Sidereal time is “star time” — it tracks the rotation of the sky relative to the stars rather than the Sun. We start from Greenwich Sidereal Time (GST) for the UT instant, then rotate east by the birth longitude to get the Local Sidereal Time. LST is the engine of the whole chart: it tells us which slice of the zodiac is currently on the meridian and horizon.

LST°  =  (GST_hours × 15  +  longitude)   mod 360

· GST_hours … Greenwich sidereal time in hours
· × 15       … convert hours of rotation to degrees (360° / 24h)
· longitude  … east-positive, in degrees

The Midheaven is where the ecliptic crosses the meridian — the highest point the zodiac reaches, due south. In equatorial terms, the LST is the Right Ascension of the Midheaven (RAMC). Projecting that onto the ecliptic (tilting by the obliquity ε) gives the MC's zodiac longitude:

RAMC = LST°
MC    = atan2( sin(RAMC),  cos(RAMC) · cos(ε) )

then normalise MC to 0°–360°

Worked example. Take LST = 60° and ε = 23.4393°:

MC = atan2( sin 60°,  cos 60° · cos 23.4393° )
   = atan2( 0.86603,   0.5 · 0.91748 )
   = atan2( 0.86603,   0.45874 )
   = 62.07°

62.07°  →  Gemini (60°–90°)  →  ≈ 2°04′ Gemini

The Ascendant is the degree of the zodiac rising on the eastern horizon. It is the most personal point in the chart and the one most sensitive to birth time and latitude, because it depends on both the LST and the observer's latitude φ:

Asc = atan2(  cos(LST),
            −( sin(LST) · cos(ε)  +  tan(φ) · sin(ε) )  )

then normalise Asc to 0°–360°

· LST … local sidereal time (degrees)
· φ   … geographic latitude (north-positive)
· ε   … obliquity of the ecliptic ≈ 23.4393°

The tan(φ) term is what makes the Rising sign latitude-dependent: near the equator it is small, but at high latitudes it grows quickly, which is why some signs rise for a very long time and others barely at all.

Worked example. Same LST = 60°, at latitude φ = 40°N:

Asc = atan2(  cos 60°,
            −( sin 60° · cos 23.4393° + tan 40° · sin 23.4393° ) )

    = atan2(  0.50000,
            −( 0.86603 · 0.91748 + 0.83910 · 0.39778 ) )

    = atan2(  0.50000,  −( 0.79456 + 0.33377 ) )
    = atan2(  0.50000,  −1.12833 )
    = 156.10°

156.10°  →  Virgo (150°–180°)  →  ≈ 6°06′ Virgo

Note the result lands in quadrant II (positive y, negative x), which is why atan2 — not a plain arctan — is essential: it keeps the angle in the correct quadrant across all 360°.

For the same UT instant, an ephemeris gives each body's ecliptic longitude directly. Each longitude is converted to a sign and degree with the same divide-by-30 rule from earlier, then dropped onto the wheel. The Sun and Moon are included alongside Mercury through Pluto. Your “Big Three” is just three of these results: the Sun sign, the Moon sign, and the Ascendant you computed in Step 4.

Houses divide the wheel into twelve life arenas. There are several systems; two are worth knowing:

• Whole-sign houses. The simplest and oldest: the sign containing the Ascendant becomes the entire 1st house, the next sign the 2nd, and so on. Clean and symbolic — each house is exactly one sign.
• Placidus. A time-based system that divides the path each degree takes between horizon and meridian. House sizes become unequal and it relies directly on the MC and Ascendant you already calculated. It is the most common modern system.

With whole-sign houses, the house a planet occupies is found by counting signs from the Rising sign:

house = ((planetSignIndex − risingSignIndex + 12) mod 12) + 1