Geddy: Vitamin D Sun Tracker

Today's vitamin D window: when it opens, how many minutes to stay out, and an alert that stops you before you burn.

WHY THE SUN'S ANGLE, NOT JUST THE UV INDEX

Most sun apps show you a UV index and stop there. A UV index cannot tell you whether the sun is making any vitamin D at all. Below about 45 degrees of elevation the atmosphere absorbs most of the UVB before it reaches your skin, and synthesis collapses. Geddy computes the sun's altitude for your coordinates, so it can say the window is open, that it opens at 10:40, or that at your latitude in January it is not coming at all.

HOW LONG, AND WHY THAT LONG

One number on the Today screen: the minutes to go out for. Underneath it, the reason - your daily goal, or your burn ceiling. Those are different decisions when the minutes are up.

The estimate uses your Fitzpatrick phototype, your age and how much skin is exposed, set per session because a t-shirt and a swimsuit are not the same input.

THE CEILING INTERRUPTS YOU. NOTHING ELSE DOES.

Every session carries a hard limit from the minimal erythemal dose for your skin type at the current UV index. Reach it and Geddy sends a time-sensitive notification that breaks through Focus, because the cost of that alert being silenced is a sunburn. It reschedules as the UV changes, always toward stopping sooner.

No ad, no banner and no upsell ever appears while a session runs. A Live Activity keeps elapsed time, dose and time left on your Lock Screen and in the Dynamic Island.

YOUR LOG, AGAINST YOUR BLOOD WORK

Every session with its duration, estimated IU and bounds, the UV it ran under and where, with totals by day and week, goal attainment and streaks. Record your serum 25(OH)D results and chart them against months of estimated intake: the estimate is a hypothesis, the lab result is the measurement, and it should correct the model.

Sessions are written to Apple Health as Time in Daylight. Geddy only writes, and asks for no read access at all.

HONEST ABOUT WHAT IT KNOWS

The dose figure is a model output and Geddy marks it provisional wherever it appears, because its coefficients are not yet traced to a published source. It shows bounds, not one confident number. If the weather service cannot be reached it says so, rather than inventing a UV index.

PRIVATE, AND ON ALL YOUR DEVICES

There is no sign-in. Your sessions, dose history and blood results live on your iPhone and sync through your own iCloud account, so they survive a lost phone. They reach no server of ours, because there is no server of ours, and health data is never sold or shared. Set your position by hand and never grant location access at all; otherwise your coordinates go to Apple's weather service, and nowhere else.

One card on the Profile screen shows apps by other independent makers. That is the only advertising in Geddy, and it is never given your health data.

THREE SESSIONS IN TWO WEEKS, THEN GEDDY PRO

The first three sessions are on us, within your first two weeks - three in total, not three a day. After that, Geddy Pro runs the session: the timer, the burn ceiling, and the alert to cover up. It also opens the day's arc, the hourly UV and sun-angle charts, the live readings and the almanac.

Today's window, its suggested minutes, the current UV index, the time before you would burn and every logged session stay open whether you subscribe or not.

Geddy Pro is 29.99 USD a year or 4.99 USD a month, with a one-week free trial. Prices vary by country and are shown before you confirm. Payment is charged to your Apple Account and renews unless cancelled at least 24 hours before the period ends. Cancel in iOS Settings.

Geddy is not a medical device and does not give medical advice. Never treat time remaining as permission to stay out. Talk to a doctor first if you have a history of skin cancer, a photosensitivity condition, or take medication that increases sun sensitivity.

Weather data provided by Apple Weather.