There are several ‘zmanim’ or halachic times that one must be familiar with to fully appreciate how the time restrictions on prayer work.
Amud/Alos Hashachar – Dawn, or first light. This is either 72 or 90 minutes before Neitz Hachamah
Hair Penei Hamizrach – when the whole eastern skyline is lit up. According to many opinions when the Halacha refers to ‘Amud Hashachar’ it really means this time. (See Biur Halacha beginning of siman 89).
Misheyakir – The Gemara establishes this as the time which one can differentiate between techeles (blue) string and white string on their tzitzis. This is the same time that one can recognize an acquaintance from 4 amos (6-8 feet) away. Rav Moshe Feinstein explains that this is a subjective zman, it depends on the clarity of the day and so on.
The Mishah Berura writes (58:18) that misheyakir is after the ‘hair penei hamizrach’.
Rav Elyashiv (Hearos on Brachos 9b) says this is 30 minutes before Hanetz Hachamah.
Rav Moshe Feinstein in Igros Moshe (OC 4:6) says that this is 35-40 minutes before Hanetz Hachamah, based on his observations on clear days.
Rav Tikushinsky (luach Eretz Yisrael) as well as the Kaf Hachayim (18:18) say that the minhag in Jerusalem is to consider a full hour before hanetz hachamah to be considered misheyakir. Other opinions range from 42 -52 minutes before Neitz in Jerusalem. Presumably these opinions do not hold that is a subjective zman, rather a fixed time like all the other zmanim.
Hanetz Hachamah – Sunrise. When the top of the orb of the sun is visible on the horizon. Some say the entire orb must be visible.
There is discussion amongst the Achronim if this is measured at real elevation or at sea level. [The practical difference is generally minimal].
This is all working with dawn being at a fixed time before sunrise. There is however a way of calculating by degrees.
To be continued…
i thought vasikin is saying shema at mishayikir and amidah at neitz. how can there be 35 minutes between them?