Most people start with one chart, their own, and read it fifty times. The tools get more interesting when you have twenty charts and can put them next to each other.
Here's how the library works on Astrolas.
Profiles are the unit
Every chart you save is a profile: a name, a date, a time, a place. Free accounts get as many as they want, so there's no reason to be precious about it. Save the questionable birth times too, and label them.
Two habits pay off immediately.
Put the source in the name. "Mum (birth cert)" and "Mum (she thinks ~7pm)" are different charts and you will forget which is which. A rectified time and a remembered time are not the same object.
Save charts you're studying, not just people you know. If you're learning what a Saturn-Pluto square looks like in practice, five charts that have one will teach you more than reading five delineations of it.
Bringing charts in
One at a time, use the birth chart page and save when the wheel looks right.
If you already have a list somewhere, there's a CSV import for bulk loading. It takes name, date, time and place, and resolves the timezone from the location rather than making you look up historical offsets, which is where most manual entry goes wrong. Daylight saving rules have changed a lot, and they've changed differently in different countries.
Comparing two charts
Synastry is the direct comparison. Two charts, one wheel, and every aspect between them.
The synastry tool takes two saved profiles and draws them as a bi-wheel, natal on the inside and the second chart on the outside. The cross-aspect grid lists the contacts, so you can see at a glance whether the connection runs through the Sun and Moon, through Venus and Mars, or mostly through Saturn.
A few practical notes.
Read the house overlays as well as the aspects. Where the other person's planets fall in your houses says a lot about which part of your life they show up in, and it's a layer that a bare aspect list skips.
Watch the orbs. Cross-aspects with a five degree orb are weak and there are always plenty of them. The ones inside two degrees are the ones people actually feel.
If either birth time is shaky, ignore everything involving angles and houses in the comparison and read planet to planet only. It's less exciting and it's the part that survives.
Comparing a chart to itself over time
The other kind of comparison is one chart at two moments, and that's what the predictive tools do.
Transits put today's sky against the birth chart. Solar arc pushes the whole chart forward about a degree per year. Progressions run the day-for-a-year key. Solar return casts a fresh chart for the moment the Sun comes back to its birth degree, once a year.
All of them read from the same saved profile, so once a chart is in the library you can move between them without re-entering anything.
A workflow that works
Save the chart. Read the birth chart properly first, because every predictive technique is only bringing forward something the natal chart already promised.
Then look backwards before you look forwards. Run solar arc or transits over five years you've already lived. You know what happened in those years, so you can check whether the technique is describing your life or describing astrology in general. That check is worth more than any amount of reading about what a technique is supposed to do.
Then, and only then, look at next year.
Naming and organising
A library of forty charts is only useful if you can find the one you want.
Some conventions that hold up after a few months:
- Put the year in the name for public figures. There are several Michael Jordans and you will eventually save two of them.
- Mark rectified times explicitly. "Rectified 04:12" is honest; "04:12" pretends to a certainty you don't have.
- Keep study charts and people separate in the naming, with a prefix or a tag. "Study: grand trine" next to "Aunt Mai" gets confusing quickly.
None of this is enforced by the software. It's just what stops the library turning into a drawer.
Getting the timezone right
The most common data error in a saved chart isn't the time, it's the offset.
Historical daylight saving is a mess. Countries have started and stopped it, changed the dates, and occasionally run it year-round. Vietnam's offsets shifted several times in the twentieth century. Parts of the United States opted in and out county by county. If you enter an offset by hand based on what the country does today, charts from before about 1980 will quietly be wrong by an hour, which moves the Ascendant by about fifteen degrees.
Entering a place name instead of an offset avoids the whole problem, because the timezone database knows what the rule was on that date at that location. Do that, and check the resolved offset once against a rough expectation.
Working with an uncertain time
Save it anyway, and label it. Then use the chart in the way it can be used.
Read planets, signs and aspects freely. Those survive a fuzzy time.
Treat houses and angles as provisional, and don't build a reading on them.
If you want to narrow the time down, the predictive tools give you a way in. Take two or three dated events you're sure about, run solar arc or transits, and see which candidate birth time puts a matching contact on an angle in those years. This is rectification, it's slow, and the answer is never certain. But an hour of it beats guessing.
A note on other people's charts
Two practical points.
Ask before you read someone in depth, especially a partner or a colleague. Synastry surfaces uncomfortable things and people react differently to being analysed than they expect to.
And be careful with birth data as data. It's identifying information: date, time and place of birth is most of what a fraud check asks for. Keep a private library private, and think twice before posting a friend's chart publicly with their full name attached.