From a date and a place to a precise sky
Every star map poster on this page is generated from real astronomical math, not a decorative picture of random dots. When you choose a date, a local time and a city, the tool first converts that moment into a Julian Day, the continuous day count used by astronomers since 4713 BC. It then computes the Greenwich Mean Sidereal Time — the angle by which the Earth has rotated relative to the distant stars — using the standard Meeus formula, and adds your east longitude to obtain the Local Sidereal Time. That single angle tells the calculator exactly which part of the celestial sphere is crossing your meridian at that instant.
Each of the stars in the chart comes from the ATHYG v3.2 catalog. The embedded table holds 327 of the brightest stars across the entire sky with measured right ascension, declination and visual magnitude. For every star the tool converts its equatorial J2000 coordinates into local altitude and azimuth with the standard hour-angle transformation. Stars whose altitude is below zero are under the horizon and are hidden, so the disk always shows exactly the sky that was actually overhead at your chosen place and time — including the way familiar constellations tilt, rise and set as the night progresses.
Brighter stars are drawn larger according to their magnitude, and you can optionally add cross-shaped diffraction rays for the most luminous ones. Constellation lines connect stars with their traditional stick figures, but a segment is drawn only when both endpoints are above the horizon. The circular layout maps the whole visible hemisphere through a stereographic projection with north, east, south and west reference marks, while the rectangular layout plots the sky directly against altitude. Four color themes are available, from deep midnight blue to a light gallery style.
Print-ready, private and free
The preview you see is only a lightweight rendering. When you press Download, the poster is redrawn from scratch at 300 dots per inch in your selected print size — A4, A3, 16×20, 18×24, 24×36 inches or a 12-inch square — with the title, date, place and coordinates composed in crisp typography. Because vectors and text are repainted at print resolution, your file stays sharp when enlarged. Everything runs locally in your browser: no account, no upload, no watermark, and no server ever sees the date or location of the moment you want to remember.
从一个日期、一个地点，到一片精确的星空
本页的每一张星空海报都来自真实的天文计算，而不是随机撒点的装饰图。当你选定日期、当地时间和城市后，工具会先把这一刻换算为儒略日——天文学自公元前 4713 年起连续计日的日数；再用标准的 Meeus 公式计算格林尼治平恒星时，即地球相对遥远恒星转过的角度，并加上你所在地点的东经，得到地方恒星时。仅凭这一个角度，计算器就能确定此刻哪一段天球正经过你的子午线。
图中每一颗恒星都来自 ATHYG v3.2 星表：内置表格收录了全天最亮的 327 颗恒星，带有实测的赤经、赤纬与视星等。工具会把每颗星的 J2000 赤道坐标通过标准时角变换换算成当地的高度角与方位角；高度角低于零度的星位于地平线以下，不会被绘制。因此圆盘上呈现的，正是你选定的那个时刻、那片土地上方真实可见的天空——包括熟悉的星座随着夜色推移而倾斜、升起与落下的样子。
越亮的恒星会按其星等画得越大，还可以为最亮的星选择性地加上十字光芒。星座连线沿用传统的连线造型，但只有当一条线段的两个端点都位于地平线以上时才会绘制。圆形版式通过立体投影呈现整个可见半球，并标注北、东、南、西；矩形版式则直接按高度角绘制天空。配色提供深夜蓝、炭黑、霁蓝、素白四种主题，适合不同的装裱风格。
可直接印刷、保护隐私、完全免费
你看到的预览只是轻量渲染。点击下载时，海报会按照所选尺寸——A4、A3、16×20、18×24、24×36 英寸或 12 英寸方形——以每英寸 300 点的印刷分辨率从零重绘，主标题、日期、地点与坐标都会以清晰精致的排版写入。由于矢量图形与文字是在印刷分辨率下重新绘制的，放大后依然锐利。所有计算都在你的浏览器本地完成：无需注册、无需上传、没有水印，也没有任何服务器会知道你想铭记的那个日期与地点。
Is this star map scientifically accurate?
Yes. Star positions come from the measured ATHYG v3.2 catalog, and the sky rotation uses the Meeus sidereal-time formulas. The map is accurate to a small fraction of a degree for modern dates, which is far finer than anything visible at poster scale. Star positions are J2000; over a human lifetime the effect of precession on the chart is negligible.
这张星图在科学上准确吗？
准确。恒星位置来自实测的 ATHYG v3.2 星表，天空旋转采用 Meeus 恒星时公式，现代日期的计算误差远小于一度的零头，远超海报尺寸下任何可见的精度要求。星位为 J2000 历元；在人的一生尺度内，岁差对画面的影响可以忽略。
What time should I enter for a birth?
Enter the local clock time at the place where the event happened — the calculator treats it as local time and combines it with the city's longitude. If you do not know the exact minute, tick the “use 22:00 by default” option or simply pick an evening hour: the visible constellations change slowly through the night, so an hour or two shifts the rotation only slightly.
出生时间应该填几点？
填写事件发生地当时的钟表时间即可——计算器会把它作为当地时间，并结合城市经度计算。如果不知道具体分钟，可以勾选「时间未知时默认使用 22:00」，或直接选一个晚间时刻：可见星座在一夜之间变化缓慢，一两个小时只会让画面轻微旋转。
Will the downloaded file print well?
The PNG export is repainted at 300 DPI in your exact chosen size, so it is suitable for framing, gifting and gallery walls. For the largest 24×36 inch size the file contains about 7200 by 10800 pixels. Use genuine photo or matte poster paper rather than ordinary copy paper for the deepest blacks.
下载的文件印刷效果好吗？
导出的 PNG 会严格按你选择的尺寸以 300 DPI 重绘，适合装框、送礼和画廊墙展示。以最大的 24×36 英寸尺寸计算，文件约为 7200×10800 像素。印刷时建议使用真正的相纸或哑光海报纸，而不是普通复印纸，黑色会更加深沉。
