# Help on reading ephemeris in HJD(TT)

1. Nov 20, 2013

### Trance

1. The problem statement, all variables and given/known data

The star HD209458 has an orbiting planet. The planet has a transit with ephemeris
HJD(TT) 2453344.768245 + 3.52474859 E

I wish to view the transit of the planet from the WHT (William Herschel Telescope) in La Palma.
On which night in 2014 is the transit best observed?
What is the precise UTC of the mid-transit?

2. Relevant equations

None that I can think of (Except perhaps equations that convert units)

3. The attempt at a solution

I'm new to this terminology, and can't find any resources online that go into any detail. The planet transits with a regular period of course, and I believe the format of the above expression of the time of mid-transit is intended to convey this somehow. I had thought that the HJD(TT) 2453344.768245 part referred to a date (which it does, sometime in 2004) and the 3.52474859 E part indicates the periodicity, ie that the planet transits every ~3.5 days (in HJD(TT)), but if this is the case, there will be many nights on which the transit can be observed. Could anybody enlighten me on the correct way to read the ephemeris?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 24, 2013

### phyzguy

You are doing it correctly, and you are right that there are many times that the transit occurs. However, some will be in the daytime, and some will be when the star is below the horizon or low in the sky. You are asked to find which one is best situated for viewing from La Palma. One way might be to calculate the altitude of the star at mid-transit. The best one might be when the star is highest above the horizon.