The exact sky above the place where your baby was born
A birthday night-sky poster begins with four simple facts: the baby's name, the calendar date of the birth, the clock time if you know it, and the city — or the exact latitude and longitude of the hospital or hometown. From those facts this tool reconstructs the sky with genuine astronomical mathematics. The chosen moment is converted into a Julian Day, the continuous day count astronomers have used since 4713 BC, and then into Greenwich Mean Sidereal Time with the standard Meeus formula. Adding the east longitude of the birthplace gives the Local Sidereal Time, the single angle that determines which constellations were crossing the meridian at the minute of the birth.
Every dot on the disk comes from the embedded ATHYG v3.2 star catalog, a measured table of the 327 brightest stars across the whole sky with their right ascension, declination and visual magnitude. Each star is transformed from J2000 equatorial coordinates into local altitude and azimuth through the hour-angle equations. Stars below the horizon are simply not drawn, so the circular map — a stereographic projection of the visible hemisphere — shows precisely what shone overhead that night: the Great Bear wheeling around the pole, Orion or Scorpius riding the southern sky, and the Milky Way's bright sentinels placed exactly where they belonged. Brighter stars render as larger points, the traditional constellation stick figures can be switched on or off, and the most luminous stars can carry delicate cross-shaped diffraction rays.
If the birth time is unknown, leaving the time field empty automatically uses 22:00, a sensible evening default; the sky turns by only about one degree every four minutes, so even an uncertain hour moves the pattern barely enough to notice. You may type the baby's name to add a personal dedication, and choose one of four gallery themes from deep midnight blue to a light ivory style suitable for nurseries.
A private keepsake, ready to frame
The screen image is only a fast preview. When you press Download, the entire poster — background, star field, ring, two-line title, optional name dedication, birth date and time, place and coordinates — is repainted from scratch at 300 dots per inch in A4, A3, 16×20, 18×24, 24×36 or square 12-inch sizes. Because text and geometry are redrawn as vectors, the print stays razor sharp at any enlargement. Nothing is uploaded: no account, no watermark, no server log of the name, date or birthplace. It is a private, free, scientifically accurate heirloom made entirely in your own browser.
宝宝出生地上方，那片真实的星空
一张生日夜空海报，只需要四个简单信息：宝宝的名字、出生的公历日期、出生的钟表时间（知道就填）以及出生城市——或者医院、故乡的精确经纬度。工具会用真正的天文计算把这一刻的天空还原出来：先把选定的时刻换算为儒略日，即天文学家自公元前 4713 年沿用至今的连续计日；再用标准的 Meeus 公式计算格林尼治平恒星时，加上出生地的东经，得到地方恒星时——仅凭这一个角度，就能确定出生那一刻有哪些星座正经过子午线。
圆盘上的每一个亮点都来自内置的 ATHYG v3.2 星表：这是一张收录全天 327 颗最亮恒星的实测数据表，含赤经、赤纬与视星等。每颗星都会通过时角方程，从 J2000 赤道坐标换算为当地的高度角与方位角；位于地平线以下的星不会绘制。因此这张以立体投影呈现可见半球的圆形星图，精确还原了那晚头顶的星空：绕着天极旋转的大熊座、横亘南天的猎户座或天蝎座，还有银河两岸那些明亮的路标星，都恰好在它们应在的位置。越亮的星画得越大，传统的星座连线可以自由开关，最亮的恒星还可加上纤细的十字光芒。
如果不知道具体出生时间，留空时间栏会自动采用晚上 22:00 这一合理的晚间默认值；星空每四分钟才转动约一度，所以即使时间差上一两个小时，画面也几乎看不出变化。你可以填写宝宝姓名，加入一句专属献词，并从深夜蓝、炭黑、霁蓝到适合婴儿房的素白四种画廊主题中任选其一。
私密的纪念，装框即可悬挂
屏幕上看到的只是快速预览。点击下载时，整张海报——背景、星野、圆环、双行主标题、可选的姓名献词、出生日期与时间、地点与坐标——都会在 A4、A3、16×20、18×24、24×36 或 12 英寸方形尺寸下，以每英寸 300 点从零重新绘制。由于文字与图形全部按矢量重绘，放大到任何尺寸都依然锐利。整个过程不上传任何信息：无需注册、没有水印，也没有服务器会记录宝宝的姓名、日期或出生地。这是一件完全在你自己浏览器中完成的、私密、免费且科学准确的传家纪念。
Which time should I enter — the hospital's local time?
Yes. Enter the clock time shown at the birthplace itself, including any daylight saving that was in effect; the tool treats your entry as local wall-clock time and combines it with the city's longitude. If the exact minute is unknown, leave the time box empty and 22:00 will be used automatically. The constellations shift slowly, so an uncertainty of an hour or two changes the disk only slightly.
应该填哪个时间——医院当地的时间吗？
是的。填写出生地上当时挂钟显示的时间即可（若当时实行夏令时也按挂钟时间填写），工具会把它视为当地钟表时间并结合城市经度计算。如果不确定具体分钟，留空时间栏即自动使用 22:00。星座转动缓慢，一两个小时的误差只会让圆盘轻微变化。
My child's birthplace is a small town not in the city list. What can I do?
Choose “Custom coordinates…” in the place menu and enter the latitude and longitude of the hospital or town, for example from any map application. Positive values mean north latitude and east longitude. The map will then be centered exactly on that point, and the printed footer will show the coordinates you entered.
孩子出生在小城市，下拉列表里没有怎么办？
在地点下拉中选择「自定义坐标…」，填入医院或小镇的纬度与经度即可（可从任意地图应用读取）：正数表示北纬、东经。星图会精确以该点为中心，海报页脚也会印出你填写的坐标。
Is the poster suitable for a nursery or as a christening gift?
Absolutely. The ivory White theme is designed for light frames and nursery walls, while Midnight and Charcoal suit dramatic gallery mounts. The exported PNG is repainted at 300 DPI in your exact print size, with the baby's name, date and birthplace set in classic serif typography, making it a ready-to-frame new-baby or christening keepsake.
这张海报适合婴儿房或做洗礼礼物吗？
非常适合。素白主题为浅色画框与婴儿房墙面而设计，深夜蓝与炭黑则适合沉稳的画廊装裱。导出的 PNG 会在你选定的尺寸下以 300 DPI 重绘，宝宝姓名、日期与出生地以经典衬线字体排印，装框后即是一份出色的新生儿或洗礼纪念礼物。
