Solar arc direction
Solar arc direction moves every point in the chart forward by the same amount the Sun has progressed, roughly one degree for each year of life, so the whole chart advances together.
Because everything moves at one rate, the technique is unusually easy to work by hand: add about a degree per year to any natal position and see what it reaches. That simplicity is why it survived alongside more elaborate methods.
Its characteristic use is finding the years in which a directed planet arrives at an angle or at another natal planet. Those contacts are read as periods of emphasis rather than as dated events.
Frequently asked
How fast does a solar arc move?
About one degree a year, the Sun's own daily motion, applied to every point in the chart alike.
How does it differ from secondary progression?
Secondary progression lets each body move at its own rate; solar arc moves them all at the Sun's, so the chart keeps its shape.