Sun Exposure Calculator
Work out day length at your latitude and date, and the direct sun a spot really gets.
‘Full sun’ on a seed packet means six or more hours of direct sun, but a spot’s direct sun depends on the date (day length at your latitude) and on what stands in the way. This calculator computes the astronomical day length for your latitude and date, subtracts the hours your obstructions steal, and calls the result by its proper name: full sun, part sun, part shade or shade, with what each means for vegetables.
Obstruction hours are yours to judge: walk the spot at 9, at noon and at 3, and add up the hours it sits in shadow. The calculator handles the astronomy; you handle the trees.
How this is worked out
Day length comes from the solar geometry: the sun’s declination for the date (up to 23.44 degrees at the solstices) and your latitude set the sunrise hour angle, and twice that angle, at 15 degrees per hour, is the day. Sunrise and sunset are shown in local solar time (noon when the sun is highest); clock times differ by your longitude within the time zone and daylight saving, commonly by up to an hour, so treat them as the shape of the day rather than appointments. Direct sun = day length minus your obstruction hours, and the naming bands are the standard ones the nursery trade uses: 6+ hours full sun, 4 to 6 part sun, 2 to 4 part shade, under 2 shade.
Assumptions:
- Obstructions are counted as full shadow hours. Dappled shade under a high tree is kinder than the arithmetic: count dappled hours as half if the light visibly moves through.
- Morning versus afternoon matters beyond the total: afternoon sun is hotter and stronger. A spot with 5 afternoon hours can outgrow one with 6 morning hours in cool climates, and scorch lettuce in hot ones.
- The bands flip benefit in hot climates: at low latitudes in summer, part shade is a feature for greens, and full sun at noon is a stress to be mulched and watered against.
Worked examples
1. Latitude 40 on Jun 21, 3 hours of tree shadow. Day length 14h 51m; direct sun about 11.9 hours. Full sun with hours to spare: the deduction barely matters in midsummer at this latitude, which is why summer hides shade problems.
2. The same garden on Oct 15. Day length shrinks to about 11 hours and the sun’s lower path throws longer shadows, so the same trees steal 5 hours instead of 3: the spot drops to part sun. Fall crops are placed by the October answer, not the June one.
3. Latitude 51.5 (London) at the December solstice. Day length about 7h 50m. Nothing is in full sun by the 6 hour definition if anything at all obstructs; winter gardening there is a day length game played with hardy leaves and covers.
What the bands grow
| Band | Direct sun | Grow with confidence |
|---|---|---|
| Full sun | 6+ hrs | Tomatoes, peppers, squash, corn, beans, melons, most herbs |
| Part sun | 4 to 6 hrs | Leafy greens, roots, peas, herbs; tomatoes at the top of the band |
| Part shade | 2 to 4 hrs | Lettuce, spinach, chard, mint family, currants; summer greens in hot climates |
| Shade | under 2 hrs | Ornamentals, moss, patience; vegetables mostly refuse |
The table inverts in the far south summer: the part shade row becomes the lettuce row of choice, and full sun rows need the water budget to match.
Common mistakes
- Judging a spot in June for an October crop. Shadows lengthen as the sun drops. The honest assessment date is the middle of the crop’s growing period, not the day you happen to look.
- Counting dappled light as full shadow. Moving light under a high canopy feeds plants far better than the same hours behind a wall. Gardeners routinely underrate good dappled spots.
- Forgetting the house’s own shadow. Buildings are the biggest obstructions in most gardens and their shadow sweeps the longest in winter. North of a house (in northern latitudes) is the classic dead zone.
- Planting the sun hogs in the second best spot. Allocate full sun by need, not by bed shape: tomatoes and peppers first, leaves and roots take what remains, and everyone is fed.
Putting the result to work
Run the calculator for the solstices and the equinox to learn a spot’s year: four dates describe its seasons better than any single visit. Mark beds by band on the garden sketch and assign crops by the table when planning with the Seasonal Garden Calendar. Where a fence or wall gives you reflected light and shelter, microclimate beats arithmetic, as it should.
How much sun does my spot really get?
- Enter your latitude (your phone’s weather app or any map shows it).
- Set the date in the middle of the crop’s growing period.
- Count the spot’s shadow hours across a day at that season and enter them.
- Read the band: full sun, part sun, part shade or shade, and plant to it.
- Repeat at the solstices and equinox to learn the spot’s whole year.
What does full sun actually mean?
Six or more hours of direct, unobstructed sunlight per day during the growing season, the definition behind the packet label. It is hours of direct sun, not brightness: a spot can be bright all day under high trees and still log only three direct hours, which is part shade by the measure that matters.
Is morning or afternoon sun better?
For most vegetables in cool and temperate climates, afternoon sun delivers more energy and warmth, and a spot strong in the afternoon outperforms its raw hours. In hot climates the calculus flips for leafy crops: morning sun with afternoon shade is the premium position for lettuce and greens in summer.
Why does my sunny spot fail in fall?
Two compounding changes: the day shortens and the sun’s path drops, so shadows from the same trees and buildings stretch much further. A bed in full sun in June can be a part shade bed in October with nothing moved but the sun. Judge fall spots by a mid October calculation.
Can I grow tomatoes in part sun?
At the top of the band (5 to 6 hours, especially afternoon hours), yes: expect later ripening and somewhat lighter crops, and choose determinate or cherry types, which carry less ambition per plant. Below 4 hours, tomatoes survive but do not reward the space; give it to greens and give the tomatoes the best spot.
How do I measure shadow hours without standing there all day?
Three timed checks (about 9, noon and 3) sketch the pattern well enough: note which hours the spot is lit between checks and interpolate. A time lapse on a phone, or a day of hourly photos, settles arguments. Then run the equinox and solstice dates here to project the pattern across seasons.
Day length from standard solar declination and hour angle geometry. Exposure bands (6+, 4 to 6, 2 to 4, under 2 hours) follow the standard nursery trade definitions.
