Right Ascencion & Declination.pptx

Explanation of RA and Dec

1. Right Ascension & Declination What do I do with these?

2. Why do we care? Wikipedia is generally pretty useful, but what do I do with this information? Where and when are these objects in the sky? Stay tuned…

3. Demonstration Alt AZ Point to the location: Altitude 45  Azimuth 135  RA Dec Point to the location: Right Ascension 15:20:00 Declination 2  Local Sidereal Time: 13:30:00 Answer: 30 degrees east of zenith on the celestial equator

4. What is the point of RA & Dec These numbers are the sky’s equivalate to Latitude and Longitude Objects beyond the solar system are all but fixed Stars do more relative to us The earth’s precession also moves the whole sky 24,000 years for 1 wobble For all practical purposes fixed It is based on the earth at the center of the universe Dec of 0 is directly above the equator RA of 0 is directly above 0 degrees Longitude at midnight of the vernal equinox This whole system is based on a point on the earth in the Atlantic Ocean! It is called “Null Island”

5. What is Declination? Let’s go to the north pole and lay down. Where are you looking when you look straight up? Dec = 90  Where are you looking when you look to the unobstructed horizon? Dec = 0  So… in PA & OH we are at a Latitude of about 40 , therefore:

6. What can Declination tell us about an object? So… in PA & OH we are at a Latitude of about 40 , therefore: Polaris is 40 above the horizon (with a Dec of 90) Objects with a Dec greater than 50 are always up or circumpolar 40 if you want them 10 above the horizon We can see southern objects to a Dec of -50 -40 if you want them 10 above the horizon Easy math: Latitude = Polaris’ Altitude 90 – Latitude = Max Dec for circumpolar objects -(90 – Latitude) = Minimum Dec ever visible Max Altitude (for an object that rises and sets) = 90 – Latitude + Declination Quick Math: 50 + Dec So… You want to see Omega Centauri

7. Travel plans!! So… You want to see Omega Centauri (NGC 5139) RA = 13:28:06 Dec = -47  28’ 37” Diameter of .6  And you want it 9 above the horizon! Target horizon has a Dec of -48 -9 = -57 -(90 - ?) = -57 => ? = 90 – 57 = 33 I have to travel to a Latitude of 33 So, why can’t I see it when I visit Hatteras NC with only ocean to the South and Latitude = 35  ? It should be 7  above the horizon! -(90 – 35) = Dec of -55 That take us to Right Ascension!

8. Were you paying attention? Declination was all about North/South Right ascension is all about East/West Hour Angle is very closely related – we will get to it later If you are standing (on a boat) at Latitude 0 and Longitude 0 When you look up at the zenith what is the Declination of the objects?

9. What is Right Ascension? Right ascension  (abbreviated  RA ; symbol  α ) is the  angular distance  of a particular point measured eastward along the  celestial equator   from the  Sun  at the  March equinox  to the point in question above the earth. I prefer to think about astronomy at night, so I think about midnight and the Autumnal equinox. It is close enough for our purposes.

10. What is Right Ascension? Still on that boat at Latitude 0 and Longitude 0 It is the Autumnal Equinox at midnight UTC – Therefore Local Sidereal Time is about 00:00:00 (give or take a few minutes) Objects directly north and south at 00:00:00 UTC have an RA of 00:00:00! Objects to the east of due south or north have a RA that grows toward the eastern horizon An object due East with an altitude of 45  has an RA of 3 hours (on the horizon 6 hours) So, if I face north toward England, objects to my right have a positive Right Ascension! Objects to the west of due north or south reset to 23:59:59 and RA decreases toward the horizon Objects with a right ascension of 3:00:00 will be directly overhead in 3 hours! Local Sidereal Time allows this to work anywhere

11. RA, LST, HA and Transit timing Local Sidereal time is a bit of a technical compute A Sidereal day is about 23 hours 56 minutes and 4 seconds This is how long it takes a star to return to the exact same point in the sky But there are apps for that… No excuse to not know your LST. Hour Angle (of an object) = LST – Right Ascension (of an object) Think about it… An object with a RA of 6:00:00 and Declination equal your Latitude Be directly overhead when the LST is 6:00:00 An object with a RA of 6:00:00 and any Declination Will transit (get as high as it will ever get) when the LST is 6:00:00

12. Why can’t I see Omega Centauri when in Hatteras NC? Latitude = 35.2189887 Longitude = -75.6957 RA = 13:28:06 Dec = -47  28’ 37” (-47.47694444) Max Altitude = 90 - 35.2189887 - 47.47694444 = 7.3 degrees This will happen when LST is 13:28:06 I tend to be looking in early July and midnight July 10 @ midnight LST = 19:11:33 6 hours earlier the sun is still up and OC sets by the time it is dark! Rise: 3:40 PM, Transit 6:18 PM, Set 8:56 PM Sunset: 8:19 PM, Civil Twilight: 8:48 PM, Nautical Twilight: 9:24 PM, Dark: 10:04 PM

13. How do you find an object given RA & Dec Point to Polaris - (Dec – 90) is the degrees away from Polaris Move south (up to the zenith then down) this far Get LST (RA - LST) is the number of hours until the object transits (you are pointing to the transit point) Each hour is 15 degrees 15 * (RA – LST) is the number of degrees From the point above rotate this many degrees toward the east This is hard even now that you know how to do it!

14. Practical Use #1 With the RA & Dec it is easy to know when to look for an object If Dec > Latitude the object is always up > 40 If Dec < Latitude the object rises and sets < 40 If Dec < 90 – Latitude < -50

15. Practical Use #2 (RA+.5)/2 = Best month to view the object at 7:00 PM (standard time) Since 7:00 is generally too early (RA-1.5)/2 = Best month to view the object at 9:00 PM EST (RA-3.5)/2 = Best month to view the object at 11:00 PM EST This assumes your Longitude is about equal to 15 X Time Zone (-75 = -5 * 15) West Ohio needs an adjustment because -85 not = -5 * 15 (RA-1.5+.66)/2 – the extra 10 degrees west is 66% of a time zone’s 15 degrees This also assumes standard time (RA-0.5)/2 = Best month to view the object at 9:00 PM EDT (RA-2.5)/2 = Best month to view the object at 11:00 PM EDT Examples: M5 RA 15:19:42 (15.33) Best at 11:00 PM EDT NE PA - (RA-2.5)/2 = 6.23 (June 6) Ohio - (RA-2.5+.66)/2 = 6.73 (June 21) The .5 is because we are on the edge of a time zone

16. Okay, so that was too hard (PA) All you need to do is Pick an observing time Remember your formula I like midnight, and I don’t generally observe in the winter (RA-3.5)/2 = Best month to view the object at midnight EDT Example M5 RA 15:19:42 That is roughly 15.333 (15.3-3.5) is about 12 12 / 2 is 6 Early June – That was easy!

17. Okay, so that was too hard (OH) All you need to do is Pick an observing time Remember your formula I like midnight, and I don’t generally observe in the winter (RA-2.8)/2 = Best month to view the object at midnight EDT Example M5 RA 15:19:42 That is roughly 15.333 (15.3-2.8) is about 12.5 12.5 / 2 is 6.25 2 nd week of June – That was easy!

18. Never Visible Circum polar Rises & Sets 10:00 PM Early July 10:00 PM Late June 9:00 PM Early February Road trip

19. What are these dials on my EQ mount for? Why is one fixed and the other loose? A star with the same RA is at different points in the sky throughout the year. The Declination is always the same circle. Could they help me find objects? Absolutly

20. Manual aligning and using these circles Polar align the scope Look up the bar with the counterweights you are looking at Polaris Most mounts have a “polar scope” to facilitate this process. Point to a known star with a known RA & DEC Check the Dec to confirm it is accurate Adjust the RA circle to show the correct value Look up the RA and Dec of an object of interest Use the circles to position the scope Done!

21. Disclaimers These dials are LOW precision, but they are better than nothing Expect to be within a few degrees A finder scope is a big help or a wide field eyepiece. This only works AFTER the mount is properly polar aligned As with most aspects of astronomy; don’t expect miracles

22. Resources Equatorial mounts: an astronomer's guide | BBC Sky at Night Magazine observational astronomy - German equatorial mount - Astronomy Stack Exchange (1) Right Ascension and Declination, explained. (RA and DEC celestial Coordinates in Astronomy) - YouTube

23. Useful links Star Rise and Set Time Calculator - Codebox Software Sidereal Time Calculator For Any Date, Time and Longitude (neoprogrammics.com)