Solar arc is the technique I reach for first when someone asks "when does this happen?" Transits tell you what the sky is doing to everyone. Solar arc tells you what your own chart is doing to itself, and it moves slowly enough that a hit means something.
The one-sentence version
You take every planet and every angle in the birth chart and push them all forward by the same amount. That amount is the arc the Sun travelled by secondary progression, which works out to roughly one degree per year of life.
That's it. No planet moves faster than another. Nothing turns retrograde. A 40-year-old has a chart where everything sits about 40 degrees ahead of where it started.
Why one degree a year
Secondary progression uses a day-for-a-year key: the sky on the fifth day after you were born describes your fifth year. The Sun covers a little under one degree a day, so after 40 days it has moved about 40 degrees. Solar arc borrows that number and applies it to the whole chart.
Some astrologers use the Naibod measure instead, a fixed 0ยฐ59'08" per year, which drifts from the true solar arc by a few degrees over a long life. Astrolas defaults to the true arc and shows the Naibod figure alongside it, because the difference only starts to matter after about sixty years and you should be able to see both.
What a hit looks like
A directed planet lands on a natal planet or angle. That's the event. Because the arc moves about a degree a year, and most astrologers work with a one degree orb, a contact stays live for roughly two years and peaks somewhere in the middle.
Here's the part people miss: the contact is one-way. Transiting Saturn will square your Sun, back off, come back, and square it again over eight months. Directed Saturn crosses your Sun once, in your whole life, and then keeps going. That's what makes it useful for dating a turning point rather than a mood.
A few contacts worth knowing:
- Directed Sun to the Ascendant or Midheaven. A visibility year. New role, new job title, sometimes just people suddenly treating you differently.
- Directed Saturn to the Sun or Moon. The long grind arrives. Not disaster, but the year the bill comes due on something you'd been carrying.
- Directed Midheaven to a natal planet. The career angle picks up whatever the planet is about. Directed MC to Venus and people start paying you for taste.
- Directed Mars to the Ascendant. Physical push. Often a move, a fight, or the year someone finally quits.
Reading it without fooling yourself
Three habits keep solar arc honest.
Use a tight orb. One degree is generous already. If you let it run to three you'll find a hit for every year of your life, and a technique that explains everything explains nothing.
Check the birth time before you trust an angle. The Ascendant moves about a degree every four minutes, so a birth time that's off by fifteen minutes puts your directed Sun-to-Ascendant year in the wrong place by nearly four years. Planet-to-planet contacts survive a fuzzy birth time. Anything touching the Ascendant or Midheaven does not.
Read the natal promise first. Directed Jupiter to Venus doesn't invent a marriage out of nothing. It brings forward whatever Venus was already doing in the birth chart. If natal Venus is squeezed between Saturn and Pluto in the 12th, the Jupiter year is going to feel different than it does for someone with Venus loose in the 5th.
Solar arc next to the other predictive tools
Transits are weather. Fast, frequent, sometimes loud, and they hit everyone with a similar chart at the same time.
Secondary progressions are growth. The progressed Moon takes about two and a half years per sign and describes the emotional chapter you're in.
Solar arc is structure. One pass, one direction, and it tends to correlate with the events you'd list if someone asked you to write your own timeline.
Most working astrologers run all three and look for the years where they agree. A solar arc contact with no transit backing it up often passes quietly. A solar arc contact that lands in the same season as a hard outer-planet transit is usually the year that ends up in the story.
Trying it on your own chart
The solar arc tool draws the directed positions around your natal wheel and lists every contact inside orb, sorted by date. Set the target year, look at what's within a degree, then go back to the birth chart and read what that planet was already promising.
Start with the last five years rather than the next five. You already know what happened. If the technique describes years you lived through, you'll trust it for the ones you haven't.
Worked example
Take a chart with the Sun at 12ยฐ Leo and Saturn at 3ยฐ Scorpio. Natal Saturn squares the Sun by nine degrees, which is far too wide to count as an aspect. On paper these two planets have nothing to do with each other.
Now direct. Saturn needs to travel from 3ยฐ Scorpio to 12ยฐ Scorpio to square the natal Sun exactly: nine degrees, so roughly age 9. Directed Saturn squares the natal Sun somewhere around the ninth or tenth year, and stays inside a one degree orb for about two years either side.
That's the thing solar arc does that nothing else does. It takes a pair of planets that never aspect each other natally and gives them one meeting, dated. In this chart, a Saturn-Sun conversation that the birth chart never had happens once, in childhood, and then never again.
Run the same arithmetic for every pair and you get a list of the years where two parts of the chart finally talk to each other. That list is short, which is exactly why it's useful.
The mistakes that cost people years
Directing to the wrong thing. You direct the chart and read the contacts against the natal positions. Directed planet to directed planet is a different technique and mostly noise, since the whole chart moves together and those angles never change.
Forgetting the angles move too. The Midheaven and Ascendant get the same arc. Directed MC to natal Venus is one of the most reliable career-and-money contacts in the technique, and it's easy to miss if you only direct the planets.
Reading a contact as a promise of one specific event. Directed Uranus to the Ascendant does not mean "you will move house in 2027". It means a year where the Ascendant, the whole front-of-life interface, gets shaken. Move, haircut, coming out, quitting, an accident, a new city. The category is reliable. The specific event is not, and anyone who tells you otherwise is guessing with confidence.
Ignoring which house the directed planet moved into. Ingresses matter. Directed Sun changing sign, or crossing a house cusp, is a slower and often bigger marker than a single aspect. It happens roughly every thirty years for the sign change, so when it does, pay attention.
When solar arc says nothing
Some years have no contacts inside orb. That's not a failure of the technique, and it's not a prediction of a boring year.
It means the chart's internal structure isn't being rearranged. Transits still happen. Progressions still move. Life still does things. Solar arc just isn't the tool that describes that particular year, and a good reading says so rather than stretching an orb to three degrees to find something to talk about.
The empty years are part of why the technique is credible. A method that always has something to say about every year isn't measuring anything.