Right outsideSky
Why your noon shadow more than doubles from September to December
When: September to December
Between the September equinox and the December solstice the sun's highest point of the day drops from about 47 degrees above the horizon to about 24 here. Your shadow at midday goes from a little shorter than you are to more than twice your height, and you can measure the whole slide on your own driveway with a stick and a tape.
Why this time of year
The equinox is the moment the midday sun starts dropping fastest, and it keeps falling until the solstice on December 21. A mark chalked near the equinox and another in mid October will already be obviously different, so starting in September gets you the whole slide instead of just the bottom of it.
How to notice it
- Pick a flat spot that will stay sunny all autumn and stand something upright in it: a broom handle, a garden stake, a person standing straight.
- Do it at solar noon, not clock noon. Around here the sun peaks about 12:38 in the afternoon on daylight time in late September and about 11:44 in the morning on standard time at the solstice, so either look up the day's transit time or simply watch for the moment the shadow is shortest.
- Chalk the tip of the shadow, write the date beside it, and measure from the base of the stick to the mark. Divide that by the height of the stick and you have a ratio you can compare across dates.
- Come back every two or three weeks up to December 21. Near the equinox the shadow runs about 0.9 times the object's height; at the solstice about 2.2 times. After the solstice it starts shortening again, so that is where the record ends.
Try this with kids: Have someone chalk your shadow at lunchtime. Whose is longer today, yours or the mailbox's? Come back in a month and see if your shadow still fits on the driveway.
the tilt of the Earth swings our half of the planet away from the sun through autumn, dropping the midday sun lower each week
Safety
- Never look at the sun, not with bare eyes, not through sunglasses, and not through a phone camera or binoculars.
- This measurement never requires looking at the sun. Face away from it and look at the shadow on the ground.
- Pick a spot away from moving cars, and watch for ice on the pavement on the late readings.
Sources
- aa.usno.navy.milUS Naval Observatory altitude and azimuth for the sun at 42.4N on the 2026 December solstice: maximum altitude 24.2 degrees. Verified in the raw page by the refute pass, not from a summary.
- aa.usno.navy.milUS Naval Observatory for the same location around the September equinox: maximum altitude 47.3 degrees. The roughly 23 degree drop between the two dates is the whole card.
- aa.usno.navy.milUSNO: upper transit, meaning solar noon, at 11:44 local standard time on the solstice, 16 minutes BEFORE clock noon.
- aa.usno.navy.milUSNO: upper transit at 12:38 local daylight time in late September, 38 minutes AFTER clock noon. Together with the solstice figure this shows solar noon swinging nearly an hour across the autumn, which is why the card tells people not to use the clock.
- people.tamu.eduKevin Krisciunas, Texas A&M: tan(maximum elevation) equals gnomon height divided by minimum shadow length. This is the relation used to turn 47.3 and 24.2 degrees into shadow ratios, and the refute pass recomputed both independently: 1/tan(47.3) = 0.9228 and 1/tan(24.2) = 2.2251.
- gml.noaa.govNOAA general solar position calculations: declination is derived from day of year alone, with no temperature term anywhere, and solar noon is set by longitude within the time zone plus the equation of time. This is the formal basis for the `fixed` phenology tag.
Checked against these sources on September 6, 2026.
In the guide for September, October, November, December