kbwen Lab

Sundial Simulator

Drag time, latitude, and date to watch how the sun casts a shadow on the dial.

0 min
EoT
Sun Current shadow Today's shadow-tip path Sunbeam
Solar time 12:00
Sun altitude 45.0°
Sunrise · Sunset (solar time) 06:00 · 18:00
Shadow × pin 0.47
Seasonal tilt +23.4°

Why does a sundial teach astronomy?

A sundial encodes three things in a single shadow: time (the hour angle), latitude (the dial's tilt sets shadow length), and axial tilt (summer and winter solstice arcs diverge by up to 47°). Compare mode makes this visible in seconds — something 15 years of textbook diagrams rarely achieve.

The Equation of Time explains why solar noon can differ from clock noon by up to ±16 minutes across the year. Two causes: Earth's elliptical orbit (Kepler's 2nd law speeds it near perihelion) and the axial tilt (23.45° angle between the ecliptic and celestial equator). Together they trace an analemma — a figure-8 path the sun makes at the same clock time each day.

This simulation uses a horizontal sundial model. At extreme latitudes (>70°) during polar day the sun never sets, so shadows spiral — shown with a status label. Near the equator (<5°), the shadow may cross the dial center (the sun passes overhead), which is astronomically correct.

Quick guide

Use cases, answers, and nearby tools

Compact below-tool notes that help first-run users and repeated visitors move faster without changing the main interface.

Chinese search: 日晷 模擬、太陽 影子 角度、軸傾角 互動、日晷 原理、太陽幾何

How to use

Run a clean first pass

  1. Drag the latitude slider (or pick a preset city), then choose a date.
  2. Watch the shadow sweep across the dial; enable 24× speed for a full day in seconds.
  3. Toggle Compare solstices — two ghost arcs appear showing Earth's axial tilt in action.

Examples

Real jobs this page helps with

  • Axial tilt insightAt 25°N with Compare mode on, the summer-winter shadow gap makes Earth's 23.45° tilt viscerally clear in one glance.
  • Arctic extremesDrag to 80°N in June and the shadow orbits endlessly — polar day geometry explained in motion, no textbook required.
  • Equation of time curiosityNotice the small corner dial and discover the 16-minute solar-vs-clock drift that accumulates across the year.

FAQ

What people usually want to know

Why use a sundial to learn solar geometry?
A sundial encodes three things in one shadow: time, latitude, and axial tilt. It makes abstract astronomy tangible.
What is the Equation of Time?
Earth's elliptical orbit and axial tilt cause solar noon to drift from clock noon by up to ±16 minutes across the year.
What is my city's latitude?
Taipei ~25°N, Kaohsiung ~22.6°N, New York ~40.7°N, London ~51.5°N, Sydney ~33.9°S.

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