Every sunrise and sunset time you have ever seen is a calculation, not a measurement. Someone had to decide what "sunrise" means, pick a location, and run the math. When two apps disagree by ten minutes — and they will — it is usually because they used different definitions. This guide walks through the science behind sunrise-sunset calculations, the exact inputs that matter for photographers, and how to read the numbers with confidence.
Why "sunrise time" needs a definition
The sun is a disc, not a point, and the horizon is usually not flat. "Sunrise" could mean the first sliver of the disc appearing, the disc's center crossing the horizon, or the moment the whole disc is visible. Those moments can differ by several minutes.
The standard used by most astronomical calculators — and by this site — is the moment the sun's upper limb (the top edge) crosses the horizon, with the observer's eye at sea level, refraction bending the sun's light by 34 arc-minutes, and the sun's radius of 16 arc-minutes accounted for. In practice, all of that is equivalent to: sunrise happens when the sun's center is 0.833° below the horizon. That single number is the definition behind almost every sunrise time you see.
The geometry of sunrise
To calculate when the sun reaches that position, the calculator must know where the sun is in the sky at every moment. The sun's path is determined by the Earth's rotation and its orbit — both well-known mathematically. The calculation works in four steps:
- Solar position. From the date and time, compute the sun's declination (its latitude in the sky) and right ascension. This uses a model of the Earth's elliptical orbit and axial tilt.
- Hour angle. Given your longitude, find the hour angle — how far east or west of your meridian the sun sits — for the moment the sun's center is at −0.833°.
- Local mean time. Convert that hour angle into your local mean solar time.
- Timezone offset. Add your timezone's offset to get clock time. This is where longitude within a timezone, and daylight saving, enter the picture.
The same machinery gives sunset (mirror calculation), solar noon (the midpoint), and daylight duration (the gap between them).
What the calculator actually does
This site's sunrise-sunset tool runs the standard NOAA solar calculations entirely in your browser — no server, no API, no data sent anywhere. Given a date, latitude, longitude and timezone, it returns:
- Sunrise and sunset in your local clock time, to the minute.
- Solar noon — the exact moment the sun crosses your meridian, useful as a symmetric reference point.
- Daylight duration — the total time the sun is above the horizon.
- A day/night timeline showing where in the 24-hour day the light lives.
Because the math is identical to what big astronomy libraries use, the results agree with professional almanacs to within a minute for any location on Earth.
The four inputs that matter
- Latitude and longitude. Even 10 km of movement can shift sunrise by 30–60 seconds. Use a precise point — your actual shooting location, not the nearest city. The geolocation button fills this for you.
- Timezone. The tool needs to know how your location maps to clock time, including any DST rules in effect on that date. The browser detects yours automatically; when planning a trip, override it to match the destination.
- Date. Sunrise moves every single day. Between the equinox and solstice it can shift 3 minutes per day at mid-latitudes — planning "last week's time" can miss the light by half an hour in a month.
Why your phone and this tool disagree
Differences of a few minutes come from definitions, not errors:
- Horizon altitude. Some apps assume a flat sea horizon; this site does too (the standard). If you are in mountains, add an obstruction angle — see our mountain guide.
- Atmospheric model. Some apps use −0.833°; a few use −0.567° (ignoring the sun's radius) or even 0°. Each choice shifts times by minutes.
- Geocoding. If the app snapped you to a city center rather than your GPS point, the longitude error alone can move times by a minute or more.
- DST handling. Apps occasionally apply the wrong DST rule for a historical or future date.
When in doubt, check solar noon: it is the most robust number and should sit halfway between sunrise and sunset. If it does not, the app is using an inconsistent model.
Daylight as a photographable resource
Photographers tend to treat daylight duration as an abstraction, but it is a resource you can budget. A winter day with 9 hours of daylight and 6:30 sunrise gives you one clean golden window in the morning; a summer day with 15 hours spreads golden light across a long, relaxed evening. Knowing the daylight duration for your location tells you how many usable windows exist, how much time you have between them, and when to stop shooting and hike out safely.
For planning trips, compare daylight across candidate weeks: the week whose golden hours match your work schedule and whose daylight length gives you margin is the week to book.
你见过的每一个日出日落时间都是计算出来的，而不是测量出来的。总有人需要决定“日出”意味着什么、选取哪个地点、运行哪些数学。当两个应用相差十分钟——它们一定会——通常是因为采用了不同定义。这篇指南带你走过日出日落计算背后的科学、对摄影师真正重要的输入项，以及如何自信地解读数字。
为什么“日出时间”需要定义
太阳是圆盘而非点，地平线通常也不是平的。“日出”可能指圆盘的第一缕边缘出现、圆盘中心越过地平线，或整个圆盘可见。这些时刻可能相差数分钟。
大多数天文计算器——以及本站——采用的标准是：太阳上缘（顶部边缘）越过地平线的时刻，观测者眼睛位于海平面，折射把阳光弯曲 34 角分，并计入太阳 16 角分的半径。实际效果等价于：太阳中心位于地平线下 0.833° 时即日出。这个单一数字就是几乎所有日出时间背后的定义。
日出的几何
要计算太阳何时到达该位置，计算器必须知道每一时刻太阳在天空中的位置。太阳路径由地球自转与公转决定——两者在数学上都已知。计算分四步：
- 太阳位置。根据日期与时刻计算太阳赤纬（在天空中的“纬度”）与赤经。这使用地球椭圆轨道与地轴倾角的模型。
- 时角。给定你的经度，求出太阳中心位于 −0.833° 时刻的时角——太阳相对你经线偏东或偏西多远。
- 地方平时。把时角转换为你的地方平太阳时。
- 时区偏移。加上你时区的偏移得到钟表时间。经度在时区内的位置与夏令时从这里进入。
同一套机制给出日落（镜像计算）、日中（中点）与日照时长（两者之差）。
计算器实际做了什么
本站的日出日落工具在浏览器中完整运行标准 NOAA 太阳计算——无服务器、无 API、任何数据都不发送出去。给定日期、纬度、经度与时区，它返回：
- 日出与日落，精确到分钟的本地钟表时间。
- 日中——太阳精确经过你经线的时刻，可用作对称参照点。
- 日照时长——太阳位于地平线上的总时间。
- 一张昼夜时间轴，显示光线在 24 小时中的位置。
因为算法与大型天文库相同，结果与专业天文年历在任意地点误差都在一分钟以内。
四个关键输入
- 纬度与经度。即使移动 10 公里，日出也可能偏移 30–60 秒。使用精确点——你的实际拍摄位置，而非最近城市。定位按钮会替你填好。
- 时区。工具需要知道你的位置如何映射到钟表时间，包括该日期生效的任何夏令时规则。浏览器自动检测你的时区；规划旅行时，覆盖为目的地时区。
- 日期。日出每天都在移动。春秋分到二至日之间，中纬度每天可偏移 3 分钟——用“上周的时间”规划，一个月就可能错过半小时的光。
为什么手机与本工具结果不同
几分钟的差异来自定义而非错误：
- 地平线高度。有些应用假设平坦的海平面地平线；本站同样如此（标准做法）。如果你在山区，请加上遮挡角——见我们的山体指南。
- 大气模型。有些应用用 −0.833°；少数用 −0.567°（忽略太阳半径）甚至 0°。每种选择都会让时间移动几分钟。
- 地理编码。如果应用把你吸附到市中心而非你的 GPS 点，仅经度误差就可能让时间偏差一分钟以上。
- 夏令时处理。应用偶尔对历史或未来日期应用错误的夏令时规则。
有疑问时，核对日中：它是最稳健的数字，应位于日出与日落正中。如果不是，说明该应用使用了不一致的模型。
把日照当作可拍摄的资源
摄影师往往把日照时长当作抽象概念，但它其实是可以预算的资源。冬季 9 小时日照、6:30 日出，给你清晨一个干净的黄金窗口；夏季 15 小时日照把黄金光铺满整个从容的傍晚。了解你所在地的日照时长，就知道存在多少个可用窗口、窗口之间有多少时间、以及何时该收工安全下山。
规划旅行时，比较候选周的日照：黄金时刻与你的日程吻合、日照长度留有余量的那一周，就是该预订的周。
