How the moon phase is calculated
This poster does not use a lookup table of moon pictures: the phase for your chosen date is computed from astronomical algorithms. The date is converted to a Julian Day and passed through the truncated Meeus series for both the Sun and the Moon — low-precision formulae that return geocentric apparent longitude, latitude and distance. From those positions the tool forms the Sun–Moon angular separation as seen from Earth, then the phase angle at the Moon itself, which is the angle between the directions toward the Sun and toward the Earth. The illuminated fraction follows directly as one plus the cosine of that angle, divided by two, while the eight named phases are assigned from the ecliptic longitude difference in 45-degree bands: new moon, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter and waning crescent.
How the terminator is drawn
The boundary between sunlight and darkness — the lunar terminator — is rendered geometrically rather than faked. The dark side is painted first as an earthshine disk. Clipped inside the lunar circle, the lit side is composed from a semicircle on the waxing or waning limb plus a half-ellipse whose horizontal radius equals the disk radius times the cosine of the phase angle: for crescent phases the ellipse bows back toward the lit limb so only a sliver remains, for gibbous phases it bows across the dark side until nearly the whole disk is filled, and the quarters collapse the ellipse to a straight line exactly when half the Moon is illuminated. The lit fill is drawn with a soft shadow spread along that ellipse edge, a deliberately gentle penumbra that echoes the real transition zone, and a clipped radial shading overlay supplies the sphere's limb darkening and oblique sunlight. The result is a scientifically proportioned moon, not a symmetrical crescent icon.
Your title, printed at 300 DPI
Two title lines are yours to customize — an anniversary, a wedding date caption, a baby's birth moon, or nothing at all. The poster also prints the illumination percentage, the phase name, the date and a discreet Memory Poster Studio brand line. When you download, everything is repainted at 300 dots per inch in A4, A3, 16×20, 18×24, 24×36 or square 12-inch formats, so the terminator stays smooth in print. Choose deep midnight, warm ivory or slate blue, and the whole render runs privately in your browser with no upload and no watermark.
月相是怎样算出来的
这张海报并不调用一组现成的月亮图片：你选定日期的月相，完全由天文算法实时计算。日期先换算为儒略日，再分别代入太阳与月球的 Meeus 截断级数——这是一组低精度公式，可返回地心视黄经、黄纬与距离。工具据此求出从地球看日月的角距离，再求月球处的相位角，即月球指向太阳与指向地球两个方向之间的夹角。亮面比例直接由「一加以相位角余弦之和除以二」得出；八个相位名称则根据日月黄经差按 45 度区间判定：新月、蛾眉月、上弦月、盈凸月、满月、亏凸月、下弦月、残月。
晨昏线是怎样画出来的
月亮上明暗交界的晨昏线完全以几何方式精确绘制，而不是贴图伪造。先画出代表地球反照的暗面圆盘；在月轮裁剪区内，亮面由位于盈月或亏月一侧的半圆，加上一个半椭圆组合而成：椭圆的横半径等于圆盘半径乘以相位角的余弦——蛾眉月时椭圆向亮缘方向回弯，只留下一弯亮缘；凸月时椭圆压过暗面，直至几乎满盘；上、下弦时椭圆退化为一条直线，恰好对应亮面一半。亮面填充沿椭圆边缘带有柔和的阴影扩散，刻意模拟真实的半影过渡带，再叠加一层径向明暗渐变，表现球面的临边昏暗与斜射阳光。最终得到的是比例科学的月亮，而不是左右对称的图标式月牙。
你的标题，300 DPI 印刷输出
两行标题完全由你自定义——纪念日、婚礼日期注解、宝宝的出生月亮，也可以留空。海报还会印出照明百分比、相位名称、日期，以及一行低调的 Memory Poster Studio 品牌小字。下载时，所有内容会在 A4、A3、16×20、18×24、24×36 或 12 英寸方形尺寸下，以每英寸 300 点重新绘制，晨昏线在印刷品上依旧顺滑。深夜蓝、暖象牙、板岩蓝三种配色任选；全部渲染在浏览器本地私密完成，不上传、无水印。
How accurate is the moon phase on my date?
The illumination fraction and phase come from the standard Meeus low-precision solar and lunar series, accurate to a few hundredths of the illuminated fraction and well under a degree in longitude — more than exact enough for a wall poster, and identical to the figures printed in mainstream astronomical almanacs.
我选的日期，月相算得准吗？
照明比例与相位采用标准的 Meeus 太阳、月球低精度级数计算，照明比例误差仅为百分之几以内、黄经误差远小于一度，对墙上海报而言绰绰有余，与主流天文年历上刊载的数值一致。
What time of day does it use? Does the time matter?
The date is evaluated at noon in your browser's local time. The Moon moves through its cycle over 29.5 days, so its appearance barely changes within a single day; choosing noon simply keeps the result on the intended calendar date even across time zones.
用的是当天几点？时间有影响吗？
日期按你浏览器当地时间的正午计算。月相周期为 29.5 天，一天之内月面几乎没有可见变化；取正午也能保证在跨时区时仍落在你想表达的那个日历日期上。
Can I print the ivory theme on white paper?
Yes. The ivory theme uses a warm cream field with a deep navy moon and dark serif type, designed for light stock and nurseries. Every theme exports at 300 DPI in your chosen size; matte photo paper renders the soft terminator most faithfully on any of the three palettes.
象牙白主题能印在白纸上吗？
可以。象牙白主题采用暖米色底、深藏青月轮与深色衬线字体，专为浅色纸张与婴儿房设计。三种主题都可按所选尺寸以 300 DPI 导出；在哑光相纸上，柔和的晨昏线在三种配色下都能最忠实地呈现。
