A Guide to the Sky, Telescopes, and Telescope Programs
Why Can't I see it all tonight?
For this evening's early targets, click here.
Why Does it all Change is answered here.
Almost everything that can be seen from your location on Earth is "up" tonight, if you're willing to observe from dusk to dawn. But for the few hours after sunset, you only can see a fraction of the sky and probably not all of its "best" targets.
Why Does the Sky Change?
Where are you?
The whole sky is up at all times. Everything in it is up everyday. So why can't you see it all at any time? The sun washes out the sky during the day and, of course, the earth gets in the way. The Earth does block out the Sun so that we can see the stars, but it does also block out how far to the south we can see. The closer you get to the equator on earth, the less of the sky is permanently blocked out. From San Diego at 32 degrees North Latitude, we cannot see the sky closer than 32 degrees to the south celestial pole.
What we can see from our location on earth is based on what date and time it is.
The sky is the same at the same date and time every year. The sky rotates 360 degrees in a day, most of the sky is visible between sunset and sunrise, but it takes a whole night for everything to come up. If you want to see (almost) the whole sky, simply stay up until dawn. The whole sky precesses 1 degree towards the west every day. A year is the amount of time it takes for the earth to go around the sun, and for the sky to "complete a revolution". Three hundred sixty degrees and 365 days is not a coincidence. A degree was designed/defined to represent the rotation of the sky in one day (clearly, to have trigonometry based on 365 1/4 degree would make it even harder than it is now). So the sky is pretty much exactly the same at the same time every year, but it shifts west over the course of the year.
What does change from month to month and year to year?
While most of the sky is never-changing, other than its daily and yearly shift, there are some great targets that do change. The Sun (do NOT point a scope at it) and the Moon change position.
The Moon rises about 50 minutes later every day; appearing to travel about 12 degrees east through the sky. The Moon tends to wash out the fainter deep sky objects (like the Sun washes out everything). Observers prefer moonless skies to see the faint stuff. The Moon is an excellent binocular or telescope object, but is least interesting during a full moon, because the shadows that make lunar features stand out are missing.
Contrary to popular belief, you do not need a telescope to see the planets. You do need a telescope to see details of the planets. Venus, Jupiter, Saturn, Mars and Mercury are often among the brightest objects in the sky (particularly Venus and Jupiter). Other than Mercury, the bright four are almost always visible in the sky (they are not only when the are in line with the sun (or close to in-line)). The planets' orbit cause them to move relative to the background sky and make marking their positions on permanent sky maps impossible. Weekly or Monthly sky maps tend to show their positions.
Jupiter (and its four biggest moons) and Saturn are impressive sights even in small binoculars.
The earth is in the middle of a swarm of satellites. They pass over your head continually. They are visible only during dawn and dusk, when the sky is dark and they are still in direct sunlight (it's their reflection of the sun that make them visible). There are websites that tell you where and when to look for the brighter ones (including the International Space Station, which is very bright).
Asteroids, Comets and Meteor Showers
There are generally a couple of asteroids and comets visible with larger telescopes at any given time. Like planets, their locations vary and require a weekly or daily chart (or website) to pinpoint their position. Occasionally, comets are bright enough to see with the naked eye. Observing a bright one through a telescope can be an impressive experience.
Meteor Showers occur when the earth passes through or near a comet's orbit. These intersection times are easy to predict although the intensity of the shower is less predictable. Telescopes do not help you observe meteor showers because the showers are a "whole sky" experience. Dark sites and moonless skies do help a great deal, though.
Next: Location and Equipment
Questions or comments? Email:Jeff Martin