Sunlight Checker — PLATEAU × Solar Geometry
Tap the map, get the answer. Sunlight Checker simulates hours of direct sun — shadows from neighboring buildings included — using Japan's official PLATEAU 3D city model and astronomical solar geometry. Free, unlimited, on-device.
The sun over Tokyo right now — computing…
Actual app screen (a real building in Harumi, Tokyo)
2s
From tap to result
576
Rays cast per check
12mo
Months of sunlight, charted
0
Location data sent to servers
Viewings happen on sunny afternoons.
Nobody shows you
the winter-morning light.
So we compute it — from real building data and the sun's actual path.
The listing said south-facing, but afternoons were dark
The viewing was bright — by pure luck
Come winter, the building next door swallowed the light
My balcony plants never get enough sun
01 — Tap & Result
Tap anywhere on the map — or search an address, or use your current location. Sunlight score, monthly chart and floor-by-floor comparison appear on the spot. No real-estate expertise required.
Tap the mapAddress search and current location work too
Read the resultSunlight score and monthly chart in about 2 seconds
Compare floorsSee how 1F differs from 5F, instantly
02 — City Model
Sunlight Checker uses PLATEAU — the official 3D city model maintained by Japan's Ministry of Land, Infrastructure, Transport and Tourism. Neighboring buildings keep their real heights and shapes, and rays are cast from the sun's position to detect shadow (ray casting).
03 — On-Device
The address you check might be your next home. So Sunlight Checker runs every calculation on your iPhone. Neither the places you search nor your results ever leave the device.
The only network traffic is downloading PLATEAU building data. Nothing about you is sent.
Screens
Every screen below is a real calculation at a real place — no mockups.
Case Studies
"Is this number even right?" — real examples, answered. The physics of the sun is written into every figure.
The 119° direction faces Tokyo Bay with zero obstruction. But the sun is only in this sector for 4-6 hours in the morning. The annual average of 4.9 hours is close to the theoretical maximum.
The sun rises from the east (~90°), passes through the south (180°), and sets in the west (270°). It's near 119° (ESE) only from sunrise to around 10-11 AM. Even with zero obstruction, sunlight from the 119° direction is physically limited to 4-6 hours per day.
| Month | In-sector time | Note |
|---|---|---|
| Jan | 4.0h | Late sunrise |
| Apr | 6.0h | Best month |
| Dec | 3.8h | Worst month |
| Annual avg | 5.0h | App shows: 4.9h |
The direction filter measures "sunlight visible from a window facing that direction." Even without obstruction, hours when the sun isn't in that direction aren't counted. Switch to "All directions" to see total sunlight hours.
"South-facing = great sunlight" is conventional wisdom. But the south filter only counts 135°-225°. Morning sun and afternoon western sun are excluded.
With south (180°) and 90° sector width, the count range is 135°-225°. The sun is in this range roughly from 10 AM to 2 PM — about 4 hours (winter) to 6 hours (summer).
| Mode | Winter (Jan) | Spring (Apr) | Summer (Jul) |
|---|---|---|---|
| All directions | 6-8h | 9-11h | 10-13h |
| South (90° width) | 3-4h | 5-6h | 5-6h |
The direction filter shows "sunlight entering through a window facing that direction." It's perfect for evaluating a south-facing room, but it's a different metric from total sunlight hours.
In Japan (Northern Hemisphere), the sun always passes through the south. The sun only reaches the north sector (315°-45°) briefly during summer mornings and evenings. Zero in winter is normal.
In the Northern Hemisphere, the sun rises from the east, passes through the south, and sets in the west. It only passes through the north sector (315°-45°) just after sunrise and before sunset around the summer solstice.
| Month | In-sector time | Note |
|---|---|---|
| Jan | 0.0h | Sun never goes north |
| Jun | 1.5h | Early morning + evening only |
| Sep | 0.0h | Zero after equinox |
| Annual avg | ~0.4h | Summer months only |
Even north-facing rooms get sunlight counted in "All directions" mode from east and west. "North-facing ≠ dark all day." However, direct sunlight through a north window is virtually nonexistent.
Even surrounding buildings of only 10m can heavily obstruct 1F when the sun angle is low. 5F can clear the obstruction and secure sunlight.
Sunlight Checker shoots a ray from the observation point (selected floor height) toward the sun direction, checking if it hits surrounding buildings. Lower floors are more likely to be obstructed; higher floors get more sunlight.
| Floor | Annual avg (all dir.) | Winter (Jan) | Summer (Jul) |
|---|---|---|---|
| 1F | ~4h | ~2h | ~6h |
| 3F | ~6h | ~4h | ~8h |
| 5F | ~8h | ~6h | ~10h |
Use the floor selector to compare different floors — it helps with buying or renting decisions. Pay special attention to winter (Dec-Feb) sunlight hours.
In summer, the sun is high and building shadows are short. In winter, the sun is low and shadows from southern buildings stretch 2-3x longer. The monthly chart reveals seasonal patterns.
In Tokyo, the sun's altitude at noon varies dramatically: ~78° at summer solstice vs ~31° at winter solstice. Lower sun angles in winter mean shadows 2-3x longer.
| Season | Noon altitude | Shadow length (10m bldg) |
|---|---|---|
| Summer solstice (Jun) | 78° | ~2m |
| Equinox | 55° | ~7m |
| Winter solstice (Dec) | 31° | ~17m |
Look at the balance between orange (sunlight) and gray (obstruction). V-shape (winter dip) is most common with southern buildings. U-shape (summer dip too) means tall buildings on east/west sides. Flat means minimal obstruction.
The sunlight score is an annual average, but winter sunlight significantly affects quality of life. Focus on the "worst month" in the monthly chart.
Use Cases
From apartment hunting to everyday life — whenever sunlight matters.
Check sunlight before renting or buying. Verify an apartment's sun exposure before the viewing.
Check sunlight on your balcony or garden. Confirm the hours of sun your plants need to thrive.
Verify workspace brightness in advance and find a comfortable environment to work in.
Compare sunlight at multiple candidate locations. Make data-driven housing decisions.
Sunlight 101
How sunlight hours shape daily life — and how to read the numbers.
Sunlight time refers to the hours during which direct sunlight reaches a location in a day. It indicates the time when the sun is not hidden by clouds, excluding cloudy or rainy days. The impact of shadows from buildings and other obstacles must also be considered.
Sunlight time affects our daily life in the following ways:
In Japan, sunlight hours are longest during the summer solstice (around June 21) and shortest during the winter solstice (around December 22). In Tokyo, there's approximately a 4-hour difference between summer solstice (about 14 hours) and winter solstice (about 10 hours). When selecting real estate, it's important to consider sunlight hours during the winter solstice when they are at their minimum.
Surrounding buildings block sunlight and create shadows. Particularly when there are tall buildings on the south side, sunlight hours are significantly reduced. Sunlight Checker uses PLATEAU 3D city models to perform accurate sunlight simulation considering the height and position of surrounding buildings.
According to the Building Standards Act, it is recommended that residential living spaces (living rooms, bedrooms, etc.) ensure at least 4 hours of sunlight on the winter solstice. However, this is not a legal requirement but merely a guideline. Actual comfort varies depending on personal preferences and lifestyle.
Under the Hood
Four technologies behind every calculation.
High-precision 3D data provided by the Ministry of Land, Infrastructure, Transport and Tourism. Accurately reproduces building shapes, heights, and location information for realistic sunlight simulation.
Accurate sun position calculation based on astronomy. Calculates the sun's azimuth and altitude from latitude, longitude, and date/time, accurately reproducing seasonal and hourly variations.
Ray tracing technology in 3D space. Calculates sunlight paths and determines obstruction by buildings, computing accurate sunlight hours considering surrounding environmental impact.
All calculations completed within your device. Server-free privacy protection and fast sunlight calculation without external dependencies.
Data source: Processed data from Ministry of Land, Infrastructure, Transport and Tourism PLATEAU website (https://www.mlit.go.jp/plateau/)
Pricing
Sunlight checks have no usage cap. When you want to look deeper, there's Pro.
¥0
Free forever
¥300/mo
or ¥2,500/year (about 30% off)
FAQ
Yes, the basic features are completely free. You can use sunlight checks unlimited times without any restrictions. For more detailed analysis, please consider the Pro version (300 yen/month or 2,500 yen/year).
Available in areas where PLATEAU 3D city models are published throughout Japan. Major cities (Tokyo 23 wards, Yokohama, Osaka, etc.) are covered, and we plan to expand coverage in the future.
Using PLATEAU 3D city models from the Ministry of Land, Infrastructure, Transport and Tourism and astronomical solar orbit calculations, highly accurate sunlight simulation is possible. However, please use this as reference information. For important decisions, we recommend consulting experts.
The Pro version offers advanced features including hourly timeline, 3D shadow visualization, and directional filter. The free version also supports all floors and up to 1,000 favorites.
No. Sunlight Checker uses a server-zero architecture, and all data processing is completed within your device. We do not collect personal information (name, email address, phone number, etc.).
We calculate the sun's position using astronomical solar orbit calculations, then perform ray casting (ray tracing) using PLATEAU 3D building data to calculate sunlight hours considering the impact of surrounding buildings.
Yes. PLATEAU 3D city models include building height information, and we accurately simulate the shadow effects based on the height of surrounding buildings. You can check sunlight for any floor using the floor selector.
Free, with unlimited checks. Start with that apartment you've been eyeing.