MVNSC: Mississippi Valley Night Sky Conservation:
‘Observing the Night Sky Around Us’:
- Mississippi Valley Conservation Authority
- Royal Astronomical Society of Canada
- Ottawa Astronomy Friends
- Instructor: Pat Browne
- Assistant: Bob Hillier
- Sessions are given at the Mill of Kintail Visitor Center .
- Course sessions on Fridays: March 27, April 10, April 17, May 1 and May 8.
What’s in the Sky Tonight – Stellarium Program
- Our Local Meridian (Green line) – The North South line about our local horizon that traces through the poles and the Zenith
- The Ecliptic (Red line) that traces the plane of the Ecliptic where the planets rise and set.
The Night Sky changes as we continue to observe… The Stellarium image is a ‘snapshot’ . Here is what happens for stars and planets on our Celestial Sphere as we the Earth turns.
As stars, planets and even the Sun rotate from East to West, we see the object move upward at an angle. At its highest point, it is exactly on the meridian and after crossing the meridian it descends slowly and sets on the Western horizon.
- Observers in the Northern Hemisphere face South and look along the Ecliptic . Planets travel on the ‘path of the planets’ also called the ecliptic to cross (transit) the meridian as they reach their highest point (culminate).
- The Meridian line starts at direction North on the horizon passes through our local zenith and meets the horizon directly south.
What we see in the sky is always changing for 2 reasons
- The stars that we see in a certain part of the sky change from hour to hour because the Earth is rotating
- The stars that we see during the night change from season to season because the Earth is in orbit around the sun
- We face South in the Northern hemisphere because the Earth we stand on is always hiding half of the Sky. Planets and stars culminate at their highest point along the meridian as we face South.
- Observers at the North Pole and the Equator see a different hemisphere of stars. Observers South of the Equator will face North to see objects culminate along their meridian.
- Constellations along the swath of sky centered on the ecliptic are the 12 zodiacal constellations, like Gemini, Cancer, Leo, … most observers get to see these constellations unless they are above or below the arctic circles
Simple Stargazing as the Earth Turns
Because the Meridian is fixed to the local horizon, a celestial object (star, planet, and our own star, the Sun) will cross our meridian as the Earth spins.
We say the object “culminates” when it reaches its highest point on the meridian. Right now, in the Night Sky, Jupiter is transiting the meridian and Venus is or has already set. Jupiter and Venus are the brightest objects visible, and next comes the brightest star of winter, Sirius .
Measuring separations in the sky
- Since we are looking through our Celestial Sphere, we are measuring angles. We can use our outstretched hand to measure angles in the night sky, and hence understand size of the moon (1/2 deg) or distance from Polaris to the Bowl of the Big Dipper.
- When you look through a telescope, you see a field of view of 1 degree or less – a very small slice of sky.
The field of view is the angular diameter of the circle of sky you see when you look into the telescope eyepiece. The full Moon is about 1/2 degree wide, With the 76mm (3″) reflector we saw the moon and some space around it providing a 1 degree field.
Some ‘reflections’ from last week …
Using a simple reflector
Here is a simple reflector with a tripod and an altitude/azimuth mount.
A reflector telescope has 2 mirrors: the primary mirror which collects all the photons of light, and the diagonal secondary mirror that directs that beam of photons into your eyepiece to be magnified and marvelled at.
Aligning on Jupiter when centered in the eyepiece!
We learned two important things:
- How to line up the telescope:
- So that we can find things using the smaller spotting scope (we call it the finder)
Finders typically have cross-hairs to center a bright star. They are mini-telescopes that produce low magnification.
- You can also use other spotting devices like LED red-dot TelRads or even an arrow with flourescent paint and a small magnifying lens lined up on the optical tube.
- The principle is the same: Find the bright object, such as a first magnitude star, planet or even the moon in a low power eyepiece and then center the same object in your finder.
- You may have to adjust set screws or orient the pointer in some way
- Once the object is aligned in the center of your finder thanks to your eyepiece, you can find other objects just using your finder.
- Find things by moving the telescope using the motion controls:
- The principle is the same for adjusting your telescope altitude and azimuth.
- Use a low power eyepiece to find the object, use your coarse adjustments to move the scope in altitude and azimuth (or if you have an equatorial mount, in Right Ascension and Declination)
- Then to keep the object centered, you use fine motion control or just ‘nudge’ the telescope
We learned how the motion of the earth moves these objects out of our eyepiece or ‘field of view’
- Telescopes magnify the field of view through the eyepiece. So we have very small fields of view, and it is hard to find objects unless they are big and bright.
- We use the moon or planets to be able to sight along the telescope and find it in the small eyepiece.
- We discovered that when we aligned our finder and got Jupiter and the 4 moons in the eyepiece (the 3rd picture on the right), they drifted out of our 1 degree field of view
- What’s going on? – Is Jupiter moving so fast?
- No It is not Jupiter that is moving perceptibly in the eyepiece.. Rather, what we are observing is our own rotation on the face of the earth
- The earth rotates on its own axis in 24 hours. That represents a rotation rate of 15 degrees per hour or 60 minutes. Divide 60 minutes by 15 and we are turning at the rate of 1 degree every 4 minutes. If we are not paying attention, when we look into the eyepiece we will see that the object has indeed been carried along in the sky… or that the earth has rotated underneath our feet!
All objects that we view in a telescope will drift out of the eyepiece according to the sidereal rate. Motorized mounts to counter the earth rotation are used when we wish to track the object for imaging and spectroscopic data.
Meet the celebrities – Sirius, Regulus, Betelguese – Winter on the Western Horizon
But before we go out – Let’s look at some Observing Tips
Let’s go out and Observe the Spring Night Sky!