The sun’s noon height changes through the year by the sun’s declination, which swings between +23.44° and −23.44° and moves by up to about 0.4° a day near the equinoxes. A small-spot dish needs its tilt reset about every 6 days near the equinoxes, while a trough on a polar axis needs no seasonal adjustment at all and loses only about 8.3% of its aperture at the solstices.
Part of Tracking Systems
Seasonal angle adjustment is the slow re-tilting of a concentrator as the sun’s path moves higher or lower in the sky through the year.
How Fast The Sun’s Height Changes
The sun’s declination follows roughly a sine curve over the year, with an amplitude of 23.44°. Its steepest change is at the equinoxes: 23.44° × 2π ÷ 365 = 0.40° per day (about 0.4°). Near the solstices it changes almost not at all. Noon height is 90° − |latitude − declination| (Seasonal Sun Angle Calculation).
At latitude 40° N:
- Summer solstice: 90 − (40 − 23.44) = 73.4°.
- Winter solstice: 90 − (40 + 23.44) = 26.6°.
The parent page gives the same relations and suggests resetting every 2-4 weeks. Check that against the target.
| Interval | Maximum error near the equinox (0.40°/day) | Focus shift at f = 0.6 m |
|---|---|---|
| 6 days | 2.4° | 2.5 cm |
| 14 days | 5.6° | 5.9 cm |
| 28 days | 11.3° | 12 cm |
A dish with a 5 cm spot tolerates about 2.4° (the tracking page), so it needs resetting about every 6 days at the equinoxes. Resetting every 2-4 weeks is acceptable only for a target much bigger than the spot, or near the solstices when the change is slow.
A Trough Can Avoid The Problem
A trough tracks by turning about its long axis. If that axis is horizontal and east-west, the sun’s seasonal motion is across the trough’s aperture and must be tracked, so the trough needs re-tilting. If instead the long axis is polar-aligned (parallel to the Earth’s axis), the daily motion is tracked by turning about the axis, and the seasonal declination appears as a component along the axis, which does not move the focus off the receiver line; it only shortens the effective aperture by cos(declination). At the solstices cos 23.44° = 0.917, a loss of 8.3%. The focal line also slides along the pipe by the trough length × tan(declination), so extend the pipe slightly past the ends.
What You Need
- A tilt scale on the mount, marked in degrees
- The sun’s declination for the date (from a table or the formula)
- A level and a plumb line (Levels & Plumbs)
- A calendar of resetting dates
Method: Mark The Mount
Step 1: Compute the noon height for each reset date: 90° − |latitude − declination|.
Step 2: Calibrate the scale against the shadow of a plumb line at solar noon.
Step 3: Mark the settings on the mount for each reset date; use a pin and holes.
Step 4: Reset when the date comes, or when the focus drifts off the target at noon.
Common Mistakes
- Using the same interval all year. The change is fast at the equinoxes and slow at the solstices.
- Using the wrong sign in the southern hemisphere. Use latitude negative south, as in the sun angle page.
- Adjusting the scale and not checking the focus. The sun, not the scale, is the reference.
Safety
Adjusting the tilt puts hands near a live focus
Re-tilting a concentrator while it is uncovered can sweep the focus across your hand or leg, and ignites or burns on contact; cover the reflector before reaching in, and never look at the focus. Concentrated sunlight damages eyes permanently and painlessly.
What’s Next
- Learn next: Gravity-Driven Clock Tracker — the daily axis
- Combine with: Seasonal Sun Angle Calculation — the full formulas