国际空间站已经成为继太阳。月亮之后天空中第三亮的物体。如果你知道能在哪里看到它，发现空间站就是一件轻而易举的事，甚至，你可以找些朋友——看到了么？天的那边有个UFO。我跟你赌100块里面肯定有人类。

只是，“知道哪里能看到它”是个大前提。国际空间站绕地球公转的同时，地球也在自转。能知道在哪里看到国际空间站需要的不仅是运气，还有物理学、天文、数学建模等等知识。

但是，你知道有人在任何时刻都能告诉你国际空间站在哪。那位就是NASA。每周，休斯顿的约翰逊航天中心任务控制处都会发布全球4600个地区的观测预告，提示这些地区是否能较长时间地观察到国际空间站。今日，NASA开始一项新的服务：将观测地点信息直接发给你。你可以在“追踪空间站”系统中注册一个账号，填写自己的所在地点，并选择用邮箱还是手机短信的方式接收提醒。在卫星经过自己头顶上空的几小时之前，这个系统会自动为你发个短信。

NASA同时说：“这个系统只会在观测状况良好时提醒你。只有观测物的高度足够高（仰角40度以上），并且能够观测到空间实验室的时间足够长时才会发信息提醒。”若RP较好，估计每周能有1-2次最佳观测时机，如果RP爆低，估计每两个月才有一次机会，这取决于地球自转的位置和天空的能见度。

The International Space Station is, after the sun and the moon, the third brightest object in the sky. If you know where to look for it, you can easily see it -- no telescope required. But: if you know where to look for it. Since the Earth spins as the ISS orbits it, the station's position in the sky at any given moment -- relative to a position on land -- is hard to know for sure.

You know who always knows where the ISS is, though? NASA. Several times a week, Mission Control at the Johnson Space Center in Houston determines sighting opportunities for 4,600 terrestrial locations worldwide -- places from which the space station is visible for a long distance. Now, NASA is publicizing that list ... and sending it, in fact, directly to you. Spot the Station lets you sign up for email or text-message alerts that will let you know, a few hours beforehand, when the ISS will be passing over your area.

"This service will only notify you of 'good' sighting opportunities, NASA says -- "sightings that are high enough in the sky (40 degrees or more) and last long enough to give you the best view of the orbiting laboratory." That viewing opportunity could come as often as once or twice a week or as rarely as once or twice a month, depending on the Earth's rotation and on sky clarity. (So "don't worry," NASA says, "if there are big gaps in between sightings!")

Being, for better or for worse, pretty much the target demographic for this particular service, I just signed up for it. For Washington, D.C., Spot the Station offered location options down to the neighborhood level. And it allowed me to clarify whether I preferred to learn about morning or evening sighting opportunities. (I chose both, because why not.) We'll see how well it works. For the moment, though, the service is a nice, thoughtful feature: a way to take work that NASA is already doing ... and transform it into public wonder and goodwill.