◉ OrbitalArcs / YEAR CLOCK
REAL TIME

Astronomical
Year Clock

Explore motion.
Understand time.

DAYLIGHT

Daylight left—
Sunrise—
Sunset—
◉
AXIAL TILT23.44°

The reason for our seasons.

SUNEARTH · NOW
Heliocentric · north ecliptic pole

Ring colours: northern hemisphere

ResearchExplore
What can you use OrbitalArcs for? ↓
WHILE YOU ARE HERE, WE TRAVEL
0 km

Yet everything feels still.

LOOK. DISCOVER. INVESTIGATE.

Your place in a moving year.

OrbitalArcs reveals the patterns you experience every day: sunrise, changing daylight hours and the passing seasons.

01 / EDUCATION

Understand why things change

For pupils, teachers, families and curious visitors. Rotate Earth, play through a day or year and discover how axial tilt and sunlight shape the seasons.

  • Compare summer and winter in both hemispheres.
  • Follow the seasons lesson with multiple-choice questions.
  • Explore day, night and time with the interactive globe.
02 / RESEARCH

From observation to data

For astronomy enthusiasts, students and data explorers. Choose a time and study distance, speed and orbital position using NASA/JPL reference data.

  • Compare the Kepler model with the reference module.
  • Download a snapshot or a daily series for the year.
  • Inspect sources, methods and observed differences.

Explore: browse the planets of our Solar System. New explorations are coming soon.

Checking in each day? Track daylight duration and the change since yesterday at the top. With your location you can also see local solar information.

Why does OrbitalArcs ask for my location?

Your latitude and longitude are needed for local solar calculations. Your browser asks for permission when the site opens. Coordinates stay in this page and are not stored or sent to an external service. The year clock, lessons and research data remain available without permission.

What is live and what is a simulation?

The home page calculates using your device clock. Education can speed through a day or year. Research uses stored NASA/JPL ephemerides for the selected time. Clouds and camera motion are visual effects; sources and accuracy are explained in Model & sources and on the research page.

What time does it get dark today?

See today’s sunset for your location at the top. Twilight follows sunset: it is not immediately fully dark. Twilight duration depends in part on latitude and season. Daylight left counts only periods when the Sun is above the sunrise threshold used by the model.

How much daylight is left today?

Daylight left today counts down towards sunset during the day. Before sunrise it shows all daylight still to come today; after the last daylight interval it reaches zero. Times are approximate for your location and use your device time zone.

When do the days get longer?

In the northern hemisphere, days generally get longer after the winter solstice and shorter after the summer solstice. The pattern is reversed in the southern hemisphere. The change since yesterday shows how day length changes at your location.

Does daylight saving time change day length?

Changing the clocks does not change the Sun’s position. Sunrise and sunset do have different clock times. On the transition day, this counter follows the actual daylight intervals within your device’s calendar day.

Contact

BEHIND THE YEAR CLOCK

Real time.
An honest perspective.

The clock follows your device’s time. The orbit is calculated every second using JPL’s approximate Keplerian elements for the Earth–Moon barycentre (1800–2050). Kepler’s equation and the vis-viva equation provide the elliptical orbit and variable speed.

This is not a full Horizons ephemeris: the nominal JPL range error is about 6,000 km. Displayed values are model estimates. UTC is converted to TT using a fixed 69.184 s offset; UT1 is approximated by UTC. Seasonal events are estimates based on solar ecliptic longitude.

The orbit retains its true eccentricity (≈0.0167), with the Sun at a focus. The Sun and Earth are greatly enlarged. Month markers indicate the first day at 12:00 UTC; season colours begin at the calculated turning points. The background is a separate artistic space perspective, rather than a camera on the small orbital Earth.

The large Earth uses fixed continents, a separate cloud layer, night-time city lights, calculated solar direction and sidereal rotation. Clouds drift with an illustrative wind of 12° per day; they are not current weather. Ocean reflections, cloud shadows and twilight enhance land, water and atmosphere. The initial view faces the day/night boundary; the optional moving camera adds 0.3° per second without accelerating Earth’s physical rotation (about 15° per hour). Camera movement starts automatically; you can pause it under About this model. The atmosphere is visually enhanced. When the site opens, the app requests your location through the browser. Existing permission may allow an immediate position; the button lets you retry. With permission, solar altitude, day/night status and sunrise/sunset are calculated for your coordinates. Sunrise and sunset use a geometric solar altitude of −0.833° for the solar disc and standard refraction; terrain and actual weather are omitted. Day means above −0.833°, twilight extends to −6°, then night. Times use the device time zone, even for coordinates elsewhere. Your location is neither stored nor sent to an external service. Travel counters integrate model distances; the daily counter starts at device-local midnight.

The Chinese date comes from the built-in Intl Chinese calendar. New modules can extend the calculation engine independently of the display.

Camera movement

Camera still

NOAA · Standard sunrise and refraction ↗
NASA JPL · Model, elements and accuracy ↗
Earth and cloud textures · WebGL Earth ↗
Night map · Three.js / NASA ↗

DISCOVER OUR EARTH

Move. Look. Understand.

Northern hemisphere

Loading Earth…
View & reference lines

Axial tilt relative to the orbital plane’s normal: about 23.44°. Its screen angle depends on the camera.

Reference lines

Surface lines disappear beyond the horizon. The orbital normal is a reference direction in space, not a second Earth axis.

Drag to rotate · pinch or use +/− to zoom

10-minute lesson: why do we have seasons?

Predict → investigate → compare → explain. Work through it with a teacher, parent or classmate.

For teachers or facilitators

Learning goal: connect axial tilt to day length and opposite seasons, and distinguish this from distance to the Sun.

  1. 2 minutes: have the learner choose a prediction without revealing the answer.
  2. 3 minutes: discuss the axis, orbital normal and direction towards the Sun. Show June and December.
  3. 3 minutes: compare both hemispheres at the same latitude magnitude. Ask what stays the same and what changes.
  4. 2 minutes: have the learner explain the relationship and revisit their prediction.

Use pause and the season buttons. Explain that this discovery window can show a simulation; the year clock behind it stays live. Step times are guidelines.

Hint

Your answers stay in this page; the app neither stores nor sends them. Copy them yourself to keep them.

23.44°NDecember23.44°NJuneSUN

Schematic. Earth’s axes are parallel; dotted lines are normal to the orbital plane. Distances and sizes are not to scale.

Module: time & Earth’s axis

One rotation, different times

Earth rotates eastwards. A mean solar day lasts 24 hours: 360° divided by 24 is 15° per hour. One degree of longitude therefore corresponds to about four minutes of mean solar time.

Relative to the stars, a rotation takes about 23 hours and 56 minutes. Because Earth also orbits the Sun, the mean solar day is slightly longer.

Greenwich · mean time
Selected longitude · mean time

This is mean solar time approximated using UTC and longitude. Official time zones and daylight saving are human conventions; the real Sun also does not always keep pace with the mean Sun.

LIVE · 1×

SCHEMATIC ORBIT
Distance from the Sun
Orbital speed
Season · northern hemisphere

Clouds are illustrative. Simulation changes only this discovery window; the year clock behind it remains live.