Tidal Locking

Overview

The Moon always shows the same face to Earth. This synchronisation — called tidal locking — occurs whenever tidal forces have had time to slow a body's rotation until its orbital and rotation periods are equal. Many exoplanets in the habitable zones of red dwarf stars are expected to be tidally locked, with permanently lit and permanently dark hemispheres. This experiment makes the concept physically tangible in two versions: a simple one in which a smartphone is moved around a lamp to create a permanent day side and night side, and a phyphox file version in which two TI SensorTags measure temperature and light on the day side and the night side of a model planet.

Grades	7–10
Subject	Physics, Natural Sciences
Method	Student experiment
Duration	Variable
Language	German, English
Sensor	Simple version: smartphone light sensor (Android). phyphox file version: two TI SensorTags

Background

Tidal locking arises because the gravitational force a body experiences across its own diameter is not uniform — the near side is pulled more strongly than the far side. This differential (the tidal force) exerts a torque that progressively aligns the rotation with the orbit. The process dissipates rotational kinetic energy as heat, ultimately locking the body into a 1:1 spin–orbit resonance.

Red dwarf (M-type) stars are the most common stars in the galaxy, and their habitable zones lie close to the star — close enough that tidal locking is expected within billions of years. The planets TRAPPIST-1e (about 40 light-years away) and Proxima Centauri b (about 4.2 light-years away), both likely in the habitable zone, are leading candidates for being tidally locked. Whether such planets can sustain habitable conditions — despite having a permanent day side and night side — is an active research question.

Materials

Simple version:

• Globe lamp (e.g. a garden globe light)
• Smartphone with phyphox (Android: the light sensor is not accessible via iOS)

phyphox file version:

• Sphere with a mounting point (e.g. a model globe, a ball on a stick)
• Two TI SensorTags and a smartphone
• Globe lamp
• Adhesive tape (to mount the sensors on the sphere)

The materials listed are examples.

Setup and Procedure

Simple version (smartphone only, as in the CAPjournal article)

1. Place the globe lamp in the centre of a table and open a light-sensor experiment (illuminance) in phyphox.
2. Rotate the smartphone around the light source and/or around its own axis.
3. Create a permanent day side (one face of the phone always toward the lamp) and a permanent night side, and compare the illuminance on each side.
4. Repeat while the phone rotates freely and discuss the difference: this is how day and night arise and what synchronous rotation means.

phyphox file version (two SensorTags)

1. Tape one TI SensorTag to the day side and one to the night side of the sphere.
2. Place the globe lamp in the centre of a table and illuminate the sphere so that one side stays permanently lit.
3. Open tidal-locking.phyphox in phyphox and connect both SensorTags via Bluetooth.
4. Start the measurement. The tabs show Temperature (BMP280), IR (ambient and IR object temperature, TMP007) and Light (OPT3001, in lx) for sensor 1 (orange), sensor 2 (blue) and a combined comparison.
5. Compare the day-side and night-side curves and discuss what each means for a real planet.

Discussion

• What would the climate be like on the day side and night side of a tidally locked planet?
• The jet stream on Earth redistributes heat from the equator to the poles. Could atmospheric circulation redistribute heat from the day side to the night side?
• TRAPPIST-1e has liquid water as a possibility only if enough heat transport occurs across the terminator. What observations could confirm this?

phyphox Files

• Two SensorTags (Temperature / IR / Light tabs): tidal-locking.phyphox
• Earlier version for two SensorTags: tidal-locking_complete.phyphox

The simple version uses the phone's light sensor in any illuminance experiment. The file contains English, German and French texts. The phyphox files are maintained in the curious-signals-phyphox repository.